Add Markdown linting

[Piedone]

Just as we have linting for JS and SCSS, let's add linting for Markdown documentation files as well. Follow the example of JS and SCSS linting.

• The first step is to decide what linter to use. We want to check for coding style, as well as spelling/grammar. markdownlint looks like the best candidate for code linting but research the question:
  • It should be available on NPM and actively developed. Similarity in usage to the other linters we use is a plus.
  • See e.g. this list.
  • Check out at least the following tools too: textlint (from NPM), proselint (from NPM)
  • Maybe we'll need to combine tools.
• Tune rules based on what we have in OSOCE docs files already. Consult the Lombiq team and start polls as necessary.
• Add documentation about using this feature and configuring, as well as suppressing rules, as we have for JS and SCSS linting.
• The API should be as with JS and SCSS linting.
• Should be MSBuild-integrated, as with JS and SCSS linting.
• Is there a way to see linter violations in the IDE like with JS linting (and with SCSS linting in VS Code)? Document it.

[sarahelsaig]

Proselint is actually a Python package, The one on NPM is just a wrapper but the main library still must be installed using pip. I don't think we want another language framework dependency.

[Piedone]

Yep, we don't.

[Piedone]

Implemented in https://github.com/Lombiq/NodeJs-Extensions/pull/5.