Allow to setup Documentation pages URL for help pages redirects

What does this MR do and why?

• Contributes to #32328 (closed)
• Feature flag: #255328 (comment 692749628)

This feature was enabled

Changes

• Show a configuration option in Admin Area to set Documentation pages URL
• Redirect requests to documentation pages to the host defined by configuration option

Screenshots or screen recordings

How to set up and validate locally

1. Open http://localhost:3000/help/administration/index.md
2. Verify that documentation is rendered locally
3. Go to Admin Area -> Settings -> Preferences -> Sign-in and Help page (section)
4. Fill Documentation pages URL with value https://docs.gitlab.com/
5. Save the form
6. Open http://localhost:3000/help/administration/index.md
7. Verify that you were redirected to https://docs.gitlab.com/ee/administration/index.html

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.