Introduce markdown linter to CI

[uhbif19]

Thesis

I propose adding markdown linter to project CI. Initial discussion was here.

One possibility is to add it to Makefile, the second is to add github action, as was suggested here.

Also, there is a question which rules to enable. Linter could check all rules that not warn on current markdown state or enable just some selected rules.

Reasoning

There is at least one rule of writing project MD files that can be checked by linter, but is checked manually now.
Using linter will reduce toil.

[sobolevn]

@uhbif19 you are more than welcome! 👍

The only thing to keep in mind: it should adapt to existing spoken rules.