Align OAuth 2.0 Login/Client documentation with new format

The [OAuth 2.0 Login](https://docs.spring.io/spring-security/site/docs/current/reference/html5/#oauth2login) and [OAuth 2.0 Client](https://docs.spring.io/spring-security/site/docs/current/reference/html5/#oauth2client) reference documentation needs to be updated to align with the new format introduced in the 5.3 release.




