Add Pages Pipeline Wizard documentation

What does this MR do and why?

This MR adds the appropriate documentation for the new onboarding process for GitLab Pages. As of last week this change is rolled out to all users on gitlab.com and will land in self-managed with 15.4.

It also adds some more in-depth configuration instructions to get around the public folder issue (see the changed docs for more info).

Related instructional video: https://youtu.be/49hgxqPGofw

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.