Add man page validation to CI

[silug]

Short description

As suggested in #579 (comment), port the validate_manpage check from openfact's CI: render every man page with groff warnings enabled and fail on any diagnostic output. This catches broken roff from a bad man page regeneration (e.g. undefined macros).

Two deviations from the openfact version, both required for openvox:

• Only stderr is inspected, since some pages legitimately contain the word "warning" in their text (e.g. the disable_warnings setting in puppet.conf(5)), which would false-trip the grep-based check.
• MANWIDTH=1000 instead of 80, since some pages contain unbreakable tokens wider than 80 columns (long URLs, the ciphers default value) that produce unfixable "cannot break line" warnings.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)