Azure.Identity Conceptual Docs (with language-specific code samples)

[joshfree]

This is a tracking item to finish writing Conceptual Documentation for Identity before the November SDK update.

This is something @ellismg began last week, but it needs to be expanded with language-specific code samples as well.

Per offline discussion, this conceptual docs will be PR'd to all 4 repos and each language-specific package owner will fill-in the language-specific portion in the markdown / docs

Related: Azure/azure-sdk-for-java#6062 Azure/azure-sdk-for-net#8488 Azure/azure-sdk-for-js#5898