Docs: update Pages with Cloudflare instrunctions

From the community feedback (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30188):

I run across your comment and I think it has become partially outdated - looks like nowadays Cloudflare doesn't automatically create a Forwarding Rule that redirects *www.domain.com to https://domain.com. I created a new site with Cloudflare today and there were no Page Rules for it.

Therefore, I'm suggesting to update the relevant documentation part and instruct users to create such a redirect rule themselves.

Further Details

• Test the process (details below)
• Document any new/update process (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30786)

Test

• Project: https://gitlab.com/gitlab-pages-demos/le-for-pages
• Custom domain: https://gl-helloworld.ml/
• Certificate: by Cloudflare