Draft: Fix hardcode of "docs.gitlab.com"

What does this MR do and why?

Resolve #4009 (closed)

Remove the hardcode of docs.gitlab.com in the code.

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Recreate the GDK database

   bundle exec rake db:reset RAILS_ENV=development

2. Open the http://127.0.0.1:3000/admin/application_settings/preferences page and expect that there will no longer be hardcode in the input box