Update reactive OAuth2 docs landing page with examples

[sjohnr]

In order to highlight the configuration improvements of gh-13763, we need a place in the docs to add some reactive code examples for OAuth2 Client. We should create a landing page for OAuth2 in the reactive section of the docs that includes getting started information and some use cases with code examples. We can use reactive/oauth2/index.adoc to hold this content, and create a reactive overview page for OAuth2.

Related gh-13763, gh-13784