Add docs for Pages Access Control with custom CAs
What does this MR do?
Documents how to use GitLab Pages Access Control functionality with custom certificates issued by a custom CA
Currently using Pages Access Control on instances that have certs issued by a custom CA may encounter
Post /oauth/token: x509: certificate signed by unknown authority errors that block access and expected functionality.
The documentation addition I propose is to document a workaround for existing users with a path forward until a fix is available.
Relevant Issues: #25411 (closed) gitlab-pages#137 (closed)