Add wrapper for the markdownlint-cli2 action (OSOE-744)

[BenedekFarkas]

markdownlint-cli2 looks like the shortest path to standalone Markdown linting that doesn't rely on Lombiq.NodeJs.Extensions (which needs a solution to be built in).

The action is developed by the author of the markdownlint library, which is what Lombiq.NodeJs.Extensions uses as well.

Useful for documentation-heavy repositories, such as this one, so we don't have to wait for the OSOCE build to surface MD linting errors, similarly how this repository has its own spell checking.

Jira issue

[Piedone]

We don't need a separate issue for this, Lombiq/NodeJs-Extensions#11 covers this (that's also about having something outside of NE, it was only opened there because we initially thought it's most applicable to be done in NE). Please add this info there.

[BenedekFarkas]

I see no mention of markdown there and Lombiq.NodeJs.Extensions already does MD linting. The wrapper (and the issue) belongs in this repo, that's exactly the point of this issue (not having to use Lombiq.NodeJs.Extensions).

[Piedone]

Ah sorry, I was in the YML mindset. Yes, this is unrelated to Lombiq/NodeJs-Extensions#11 and valid here.