Allow more customization to Help landing page

What does this MR do?

• Moves custom help text to the top of the /help page.
• Allows admin to specify an alternate support URL.
• Allows admin to hide marketing-related help content that is N/A and could confuse end-users.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/31118

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

31118