Move GitLab Pages default settings

What does this MR do and why?

Moves the GitLab Pages default settings from https://docs.gitlab.com/ee/user/gitlab_com/#gitlab-pages to a new section on GitLab Pages administration.

Related issue

• Related to #347004 (closed)

Screenshots or screen recordings

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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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