Add documentation for running external GitLab Pages

What does this MR do?

Add documentation on using an external GitLab Pages instance with Charts.

Review app: http://docs-preview-charts-2276-hybrid-gitlab-pages-docs.178.62.207.141.nip.io/charts/advanced/external-gitlab-pages/index.html

Related issues

Closes #2276 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened