Add overview documentation for OAuth2

This commit adds an overview to the landing page for OAuth2 with:
* OAuth2 Resource Server
* OAuth2 Client

At this level, OAuth2 Login is combined into the overview for Client
to make it clear that it is part of the oauth2-client module.

Content is broken down into use cases similar to other docs pages
that have been updated recently. This content will be revisited
as additional updates are made to OAuth2 docs, and some use cases
may be moved or even added as needed. This commit only adds a few
initial use cases to get the OAuth2 docs update process started.

Closes gh-13784