Merge branch 'jc/doc-lint'

Find common mistakes when writing gitlink: in our documentation and drive the check from "make check-docs". I am not entirely happy with the way the script chooses what input file to validate, but it is not worse than not having anything, so let's move it forward and have the logic improved later when people care about it deeply. * jc/doc-lint: ci: validate "linkgit:" in documentation