Stocktake existing CI resources

Problem to solve

Ensure our CI documentation links to all resources written by GitLabbers and others to ensure docs.gitlab.com provides a one-stop-shop for CI information.

Further details

As a first step to fleshing out the CI documentation, lots has been written about using GitLab for CI that isn't within docs.gitlab.com.

Our CI documentation should arrange and link to these resources so that our readers can easily locate the information.

Proposal

Build on the restructure of links to external content here: https://docs.gitlab.com/ee/ci/README.html#further-resources/