Add readmes for System.Runtime, System.Runtime.Numerics, System.Buffers, System.Runtime.Intrinsics

[dakersnar]

Building off of the standard set here: #77134.

This is very much a first draft/WIP, and input is definitely appreciated. I marked areas I need input on with TODOs.

Additionally, here are some other things we might want to borrow from the reflection READMEs:

• Explanation of contribution bar, such as https://github.com/steveharter/runtime/blob/a2dcb8e9ab1b5f1ad76663c9e445c991619c496e/src/libraries/System.Reflection.DispatchProxy/README.md
• Differentiation between core-clr and mono source, if appropriate, such as https://github.com/steveharter/runtime/blob/a2dcb8e9ab1b5f1ad76663c9e445c991619c496e/src/libraries/System.Reflection.Emit/README.md
• More links to documentation.

Additionally, it is unclear to me whether or not I am correctly correlating namespaces, libraries, area labels, and source code locations. For example, the difference between System.Numerics and System.Runtime.Numerics, and the difference between System.Runtime and System. If this stuff is confusing to me, it is probably confusing to other contributors, so we should make sure we clearly explain these connections. Let me know if I've made any mistakes there.

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Issue Details

---

Building off of the standard set here: #77134.

This is very much a first draft/WIP, and input is definitely appreciated. I marked areas I need input on with TODOs.

Additionally, here are some other things we might want to borrow from the reflection READMEs:

• Explanation of contribution bar, such as https://github.com/steveharter/runtime/blob/a2dcb8e9ab1b5f1ad76663c9e445c991619c496e/src/libraries/System.Reflection.DispatchProxy/README.md
• Differentiation between core-clr and mono source, if appropriate, such as https://github.com/steveharter/runtime/blob/a2dcb8e9ab1b5f1ad76663c9e445c991619c496e/src/libraries/System.Reflection.Emit/README.md
• More links to documentation.

Additionally, it is unclear to me whether or not I am correctly correlating namespaces, libraries, area labels, and source code locations. For example, the difference between System.Numerics and System.Runtime.Numerics, and the difference between System.Runtime and System. If this stuff is confusing to me, it is probably confusing to other contributors, so we should make sure we clearly explain these connections. Let me know if I've made any mistakes there.

Author:	dakersnar
Assignees:	-
Labels:	documentation, area-Meta
Milestone:	-

[teo-tsirpanis]

Left some comments about the NuGet packages.

[jeffhandley]

I suggested breaking the tests out into their own headings where appropriate, and avoiding the "tests ... live in ..." wording with differently structured bullets instead.

[dakersnar]

@jkotas Thanks for the review, and good catch. I think this is all from a single typo that propagated via copy paste. All instances of "coreclr" should definitely be "libraries". I'll update this now.

[dakersnar]

The source of my error was copy-pasting the reference to System.Private.CoreLib from this PR, which references coreclr instead of libraries: https://github.com/dotnet/runtime/pull/76567/files.