Add README docs for XML libraries (#77401)

* Add README docs for XML libraries

* Address feedback

src/libraries/System.Private.Xml.Linq/README.md

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+# System.Private.Xml.Linq
+This assembly implements APIs for processing XML entities with LINQ operations, along with APIs to navigate and validate these entities. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+It provides core implementations of [`XDocument`](https://learn.microsoft.com/dotnet/api/system.xml.linq.xdocument), [`XContainer`](https://learn.microsoft.com/dotnet/api/system.xml.linq.xcontainer), and related types.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/linq/linq-xml-overview.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+
+## Source
+* XDocument: [../System.Xml.XDocument](../System.Xml.XDocument)
+* XPath APIs for XDocument: [../System.Xml.XPath.XDocument](../System.Xml.XPath.XDocument)
+
+## Deployment
+The LINQ to XML processing libraries are included in the shared framework and also shipped as NuGet packages. The NuGet packages are considered "legacy" and should not be referenced by projects compatible with .NET Standard 2.0.
+* [System.Xml.XDocument](https://www.nuget.org/packages/System.Xml.XDocument)
+* [System.Xml.XPath.XDocument](https://www.nuget.org/packages/System.Xml.XPath.XDocument)

src/libraries/System.Private.Xml/README.md

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+# System.Private.Xml
+This is the primary XML (eXtensible Markup Language) assembly. It provides standards-based support for processing XML.
+
+It provides core implementations of various types including [`XMLReader`](https://learn.microsoft.com/dotnet/api/system.xml.xmlreader), [`XMLWriter`](https://learn.microsoft.com/dotnet/api/system.xml.xmlwriter), [`XMLDocument`](https://learn.microsoft.com/dotnet/api/system.xml.xmldocument), [`XMLSerializer`](https://learn.microsoft.com/dotnet/api/system.xml.serialization.xmlserializer), and more. These types are exposed via the various `System.Xml[.*]` assemblies.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+
+## Source
+
+* XmlReader and XmlWriter: [../System.Xml.ReaderWriter](../System.Xml.ReaderWriter)
+* XmlDocument: [../System.Xml.XmlDocument](../System.Xml.XmlDocument)
+* XmlSerializer: [../System.Xml.XmlSerializer](../System.Xml.XmlSerializer)
+
+Higher level APIs for performing LINQ operations on XML entities:
+* LINQ to XML: [../System.Private.Xml.Linq](../System.Private.Xml.Linq)
+* XDocument: [../System.Xml.XDocument](../System.Xml.XDocument)
+* XPath: [../System.Xml.XPath](../System.Xml.XPath), [../System.Xml.XPath.XDocument](../System.Xml.XPath.XDocument)
+
+APIs to support the creation and validation of XML digital signatures:
+* System.Security.Cryptography.Xml: [../System.Security.Cryptography.Xml](../System.Security.Cryptography.Xml); for example [`EncryptedXml`](https://learn.microsoft.com/dotnet/api/system.security.cryptography.xml.encryptedxml) and [`SignedXml`](https://learn.microsoft.com/dotnet/api/system.security.cryptography.xml.signedxml).
+
+## Deployment
+The XML processing libraries are included in the shared framework and also shipped as NuGet packages.
+* [System.Xml.ReaderWriter](https://www.nuget.org/packages/System.Xml.ReaderWriter)
+* [System.Xml.XmlDocument](https://www.nuget.org/packages/System.Xml.XmlDocument)
+* [System.Xml.XmlSerializer](https://www.nuget.org/packages/System.Xml.XmlSerializer)
+* [System.Xml.XDocument](https://www.nuget.org/packages/System.Xml.XDocument)
+* [System.Xml.XPath](https://www.nuget.org/packages/System.Xml.XPath)
+
+The System.Security.Cryptography.Xml library is shipped as a NuGet package as part of .NET Platform Extensions.
+* [System.Security.Cryptography.Xml](https://www.nuget.org/packages/System.Security.Cryptography.Xml/)
+
+These NuGet packages are considered "legacy" and should not be referenced by projects compatible with .NET Standard 2.0.

src/libraries/System.Xml.ReaderWriter/README.md

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+# System.Xml.ReaderWriter
+This assembly surfaces [`XmlReader`](https://learn.microsoft.com/dotnet/api/system.xml.xmlreader), [`XMLWriter`](https://learn.microsoft.com/dotnet/api/system.xml.xmlwriter), and related types. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+
+## Deployment
+The `System.Xml.ReaderWriter` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.ReaderWriter). The NuGet package is considered "legacy" and should not be referenced by projects compatible with .NET Standard 2.0.

src/libraries/System.Xml.XDocument/README.md

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+# System.Xml.XDocument
+This assembly surfaces [`XDocument`](https://learn.microsoft.com/dotnet/api/system.xml.linq.xdocument) and related types. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/linq/linq-xml-overview.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+
+## Deployment
+The `System.Xml.XDocument` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XDocument). The NuGet package is considered "legacy" and should not be referenced by projects compatible with .NET Standard 2.0.

src/libraries/System.Xml.XPath.XDocument/README.md

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+# System.Xml.XPath.XDocument
+This assembly surfaces extensions for navigating XML documents, including the [`XDocumentExtensions`](https://learn.microsoft.com/dotnet/api/system.xml.xpath.xdocumentextensions) class. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/linq/linq-xml-overview.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+
+## Deployment
+The `System.Xml.XPath.XDocument` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XPath.XDocument).The NuGet package is considered "legacy" and should not be referenced by projects compatible with .NET Standard 2.0.

src/libraries/System.Xml.XPath/README.md

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+# System.Xml.XPath
+This assembly surfaces [`XPathDocument`](https://learn.microsoft.com/dotnet/api/system.xml.xpath.xpathdocument) and [`XPathException`](https://learn.microsoft.com/dotnet/api/system.xml.xpath.xpathexception). See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+
+## Deployment
+The `System.Xml.XPath` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XPath). The NuGet package is considered "legacy" and should not be referenced by projects compatible with .NET Standard 2.0.

src/libraries/System.Xml.XmlDocument/README.md

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+# System.Xml.XMLDocument
+This assembly surfaces [`XMLDocument`](https://learn.microsoft.com/dotnet/api/system.xml.xmldocument) and related types. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+
+## Deployment
+The `System.Xml.XMLDocument` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XMLDocument). The NuGet package is considered "legacy" and should not be referenced by projects compatible with .NET Standard 2.0.

src/libraries/System.Xml.XmlSerializer/README.md

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+# System.Xml.XMLSerializer
+This assembly surfaces [`XMLSerializer`](https://learn.microsoft.com/dotnet/api/system.xml.serialization.xmlserializer) and related types. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+
+## Deployment
+The `System.Xml.XMLSerializer` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XMLSerializer). The NuGet package is considered "legacy" and should not be referenced by projects compatible with .NET Standard 2.0.