Add commented lines explaining the file, and including
rules that should pass already, and commented out rules
that can be added in the future, for reference.

The configuration is currently only specified in CI, by moving it into
`.mdlrc` we get immediate feedback locally in supported editors.

To ensure `mdl` is available it's also added to the `Gemfile`, though
CI will still use the version installed in the `gitlab-docs-lint` image.