Linting: markdownlint-cli2

Continuing the effort started in !32 (merged) and !33 (merged) by adding a pipeline job for mdl (task listed in #11).

This MR is based on the decisions that have been made in !32 (merged) (rules, separate job for mdl, using alpine as image, etc...).
It'll also have to be rebased against !32 (merged) and !33 (merged) once they're merged.

mdl isn't packaged for Alpine (yet?), so it is installed via rubygem within the pipeline for now.

Here's a run of the mdl job.
I'd happily help to fix/treat the "issues" raised by mdl once this MR is merged! 😸

I remain available if any changes are needed 🙂