"make check-docs" noticed problems and reported to its output but
failed to signal its findings with its exit status, which has been
corrected.
* ps/check-docs-fix:
ci/test-documentation: work around SyntaxWarning in Python 3.12
gitlab-ci: add job to run `make check-docs`
Documentation/lint-manpages: bubble up errors
Makefile: extract script to lint missing/extraneous manpages