Added Azure Auth flow doc

[mshrimankar10]

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[dkarwowski]

I'm curious about @bpescato-amazon's opinion here, but I would prefer to avoid having provider-specific documentation in this repository.

There are a few cases where we have examples in the docs that refer to a GCP / AWS method for solving a problem, but not a detailed how-to for integrating with either of us.

It doesn't seem reasonable to have this repository maintain up-to-date documentation on authentication (or any other integration) for different providers (or receive issues / questions on it). On the GCP side, we have our public documentation that I'd rather refer a partner to when integrating with us.

[bpescato-amazon]

@dkarwowski Agreed that having provider specific documentation around the specifics is going to be a bit hard to maintain.

I wouldn't be opposed to having a single Authenticating.md doc or something like that where each implementing partner would be able to supply a small (2-3 sentence) description, along with a link to the relevant documentation hosted.