Update OAuth2 docs landing page with examples

[sjohnr]

In order to highlight the configuration improvements of gh-11783, we need a place in the docs to add some code examples for OAuth2 Client. Eventually, the main OAuth2 Client (and OAuth2 Login) docs will be updated and/or rewritten. In the meantime, we should create a landing page for OAuth2 that includes getting started information and some use cases with code examples. We can use servlet/oauth2/index.adoc to hold this content, and create an overview page for OAuth2.

[sjohnr]

Related gh-11783