This reworks the docs-check ci workflow in several ways:
* It renames the make target 'check-docs' to the more systematic 'check.docs'.
* It adds a 'docs'mode to the files validation script, and uses the script in `make check.docs`.
Overall, the workflow and local make targets are more systematic and
consistent with this change.
Signed-off-by: Michael Adam <obnox@samba.org>