Setup Markdown linter

[nkrkv]

Rationale

XOD docs in this repository is a code that should be formatted properly. We have Prettier, but formatting with it is not enforced. We can’t actually make pretty-printing check a part of PR validation because it is too strict, thus it will confuse people who just want to fix a typo involving a slight paragraph reflow. Such PRs should not trigger a CI error.

So, a linter should be more relaxed than the prettier check. Here is a possible solution: https://github.com/markdownlint/markdownlint

Acceptance criteria

• I can yarn lint to check everything with the linter
• Max line length is set to 100
• All existing content is tweaked to pass the linter check
• After doing yarn prettier:all the yart lint produces no errors
• CircleCI is set up and invokes the linter on PRs