www/gitlab-ce: simplify gitlab-pages documentation and add doce for it

How to configure Gitlab-Pages (currently without TLS) can be found here:
https://gitlab.fechner.net/mfechner/Gitlab-docu/-/blob/master/install/16.0-freebsd.md#9-gitlab-pages

I will check later if TLS configuration is even required if a TLS proxy
like haproxy is in front of nginx.