Documentation: add script and build target to check for broken file references

Add a simple script and build target to do a treewide grep for
references to files under Documentation, and report the non-existing
file in stderr. It tries to take into account punctuation not part of
the filename, and wildcards, but there are bound to be false positives
too. Mostly seems accurate though.

We've moved files around enough to make having this worthwhile.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>