Add a warning by danger bot to update docs whenever bot is used

Description

We created issue #477678 (closed) to update the documentation to add the list of tasks that are performed by the GitLab Admin bot. We created MR !162797 (merged) to add that information, however, the information could become stale very quickly and it is easy to miss adding the new use cases to the docs.

Proposal

To avoid missing adding new use cases to the docs we should create a danger warning similar to the one raised when sidekiq worker arguments are altered in the codebase. The danger bot would post a comment whenever GitLab bots are being used in the new MRs and would ask the MR authors to update the documentation if needed.