Efforts to reduce the resources used by CI:
* Only one git checkout
* Exclusion of irrelevant directories for several commands
* Fix some rtlcss warnings
* Move some commands from tests.yml to composer.json to make them reusasble
* Initial efforts to take avantage of all that from `make` (help welcome)
* Improved markdownlint
* Relaxed rules slighlty
* `npm run markdownlint` for automatic tests
* `npm run markdownlint_fix` for automatic syntax fixing
* Applied the fixes on all our Markdown files