Add Oauth2 plugin documentation page to rabbitmq-website

[MarcialRosales]

This PR includes :

• Add OAuth 2.0 authentication backend plugin page
• Link plugin page from the left navigation menu
• Reorganize oauth2-examples page
• Update oauth2-examples page with links to the various pages that demonstrate how to integrate rabbitmq with various Oauth2 providers
• Add section to oauth2-examples page that demonstrates Rich Authorization Requests tokens.