Add Gitlab CI docs

[AdeAttwood]

There have been a few comments on #233 on have to use commitlint with GitLab CI. Now we are crating a new top level heading on our docs for "Integrations" it would be good to get the comments from that issues into the docs.

This does depend on #2715 that is adding the main heading to the docs.

[pkoch]

I'd encourage you to not block on me and just go ahead and add that header in. I'll add my section underneath it when I'm ready. :)

[escapedcat]

Has been done, right? https://commitlint.js.org/guides/ci-setup.html#gitlab-ci