Add linter to detect broken links

Why is this change being made?

Problem

The new Handbook doesn't have a linter that verifies that links are valid. It leads to increase of broken links number.

Solution

As a quick fix, I migrate the old Handbook ruby linter to the new Handbook project.

It currently applies limited validations only, but it's better than nothing. As a follow-up, it can be improved or swapped with a different solution.