Perform link checks for markdown files that are not in the `docs` directory
There are several markdown files at the root of the repository that contain links to documentation and websites. Currently the docs
directory is automatically checked for valid links. We should include specific files that are outside this directory to catch invalid links.
- https://gitlab.com/gitlab-org/gitlab/-/blob/master/MAINTENANCE.md
- https://gitlab.com/gitlab-org/gitlab/-/blob/master/PHILOSOPHY.md
- https://gitlab.com/gitlab-org/gitlab/-/blob/master/PROCESS.md
- https://gitlab.com/gitlab-org/gitlab/-/blob/master/README.md
- https://gitlab.com/gitlab-org/gitlab/-/blob/master/CONTRIBUTING.md
Related to !134400 (comment 1612599433)
Edited by Achilleas Pipinellis