Create and test migration guide for crowd support with omniauth-generic gem

omniauth-crowd gem is currently unmaintained and is scheduled for removal in 16.0 via Cancelled: Remove omniauth_crowd gem in 18.0 (#369118 - closed) As part of recent discussion, existing users of crowd would benefit from migration instructions on using omniauth-oauth2-generic to use crowd when omniauth-crowd is removed. This task is to document and share that migration document.