Resolve "Help users find our contributing page"

What does this MR do?

Plan: https://gitlab.com/snippets/1722594

Adds a link to the user dropdown (top bar) that opens https://about.gitlab.com/contributing/

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

Why was this MR needed?

Users on both self-managed instances and GitLab.com may not be aware of our open core model.

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #47274 (closed)