Add docs exception to Danger, and fix links to foss

I noticed when making a change to .markdownlint.json that Danger complained that:

• These files couldn't be categorised, so Danger was unable to suggest a reviewer. Please consider creating a merge request to add support for them.

So, here is the MR!

Additionally, that message linked to gitlab-foss, so I updated links to gitlab within the Danger code, as needed.