Merge branch 'jc/doc-lint'

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