Clarify pages_exernal_url docs
What does this MR do and why?
People are constantly asking if it's possible to configure pages on the subdomain of gitlab, or just configuring pages in a weird way.
Some of these configurations work, but they may cause problems like:
- broken Access Control
- broken API (e.g. it worked before, but broke in GitLab 14.* when we started to use API)
Screenshots or screen recordings
These are strongly recommended to assist reviewers and reduce the time to merge your change.
How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
MR acceptance checklist
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I have evaluated the MR acceptance checklist for this MR.