Updates to the azure_identity README

[ronniegeraghty]

Frist draft for the Identity README.md.
Copied from Python identity README.md, updating syntax, examples and links.
Pretty much went through the credentials their README.md referenced and checked if we had that cred implemented yet.

Still many TODOs and ?(questions) in the doc.

[ronniegeraghty]

It looks like the checks are failing because in the README.md I put an example that uses azure_data_cosmos::CosmosClient. I believe the check is trying to run the example, but the identity crate workspace doesn't include the cosmos crate. Is that right? Not sure how to resolve this. Looks like the old README.md uses a StorageAccountClient in its example but it had to create and impl the struct in the example. I feel like that adds a lot of code to the example that users won't and shouldn't bring into their projects.

[ronniegeraghty]

Fixes #1967

[LarryOsterman]

Note to self: The markdown is NOT the content - look at the rendered output before leaving feedback.