Add network policy object for the GitLab Pages Service

What does this MR do?

Add network policy object for the GitLab Pages Service. Enhance the security of this deployment in one easy step:

gitlab:
  gitlab-pages:
    networkpolicy:
      enabled: true

An example of the ruleset that can be applied is included in our documentation.

Related issues

Closes: #2877 (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