update the Cosmos crate README

[analogrelay]

This is a first pass at updating the README for the azure_data_cosmos crate. I based this on the Go package (https://github.com/Azure/azure-sdk-for-go/blob/main/sdk/data/azcosmos/README.md) and updated the syntax and added some notes about the differences. I also borrowed from the ongoing work @ronniegeraghty is doing for the Identity create in #1990)

Please take a close look to make sure I got all the Go related stuff converted properly. I'm also doing some further editing passes, but wanted to open things up for folks so they can take some time to find errors I may have made.

[analogrelay]

We've got a bit of a build issue. The README.md file is embedded as documentation for the root of the crate, which means it builds as doctests. I think this is a Good Thing ™️ since it ensures our README samples compile properly. However, we do that build without --all-features, so it's currently failing when building the examples that use CosmosClient::with_key, since that method requires the key_auth feature. Also, I can't use any of the rustdoc "attributes" on the code block (like no_compile) to avoid that because they will break syntax highlighting on the README on GitHub (and possibly crates.io, though I'd hope they can support those attributes).

@heaths Are we able to add --all-features to the tests runs in eng/scripts/Test-Packages.ps1? I believe that's the script that's failing. I'm happy to do it, but I wanted to check in with you first.