Documentation for OAuth 2.0 client

[ikhoon]

We don't have documentation for the OAuth 2.0 client. Users should rely on Javadoc and test code to use it. It would be good to write documentation for OAuth 2.0.

• Which OAuth 2.0 specifications are supported by Armeria?
• How to create a basic OAuth 2.0 client.
• What tuning points such as retry or circuit breaker can be given to create a more reliable OAuth 2.0 client?

The API for OAuth 2.0 has been recently refactored(#5476) and I think the new API will become stable sooner or later.