Add Documentation Dangerfile

What does this MR do?

Get the danger bot to inform the Docs team every time a docs change is made.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/49551

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Conforms to the code review guidelines
• Conforms to the style guides