docs: restore po4a config + optional cmake po4a-update target - #485

Summary

PR #466 retired the autotools surface and, alongside it, deleted docs/man/po4a.config (rationale given there: "only used by docs/man/Makefile.am"). The .po / .pot files under docs/man/po/ and the committed *.LANG.1 manpages were kept, but the driver that lets translators refresh them was lost.

As @Vollstrecker pointed out in #466 (comment), po4a is not autotools-related — it's a standalone manpage translation tool that lets translators familiar with .po files keep manpage translations in sync with the English masters. Worth keeping.

This PR restores the workflow under CMake without making po4a a build dependency.

What changed

Total: +43 lines, 1 new file.

Why a translator-only target (and not a build step)

po4a was never wired into the default build under autotools either. git grep po4a against the pre-#466 tree shows the only references in build glue are:

• docs/man/po4a.config itself,
• docs/man/Makefile.am: EXTRA_DIST += po4a.config — ships the file in source tarballs, no rule invokes it,
• header comments in the generated *.LANG.1 files.

There's no AC_CHECK_PROG(po4a), no AM_CONDITIONAL, no automake rule, no Makefile recipe that runs po4a. Translators ran po4a po4a.config by hand and committed the regenerated artifacts. This PR keeps that workflow: the *.LANG.1 files remain the source of truth for install, and po4a-update is the regen helper.

When po4a is absent on the host, the target simply isn't created — no configure error, no CI impact.

Verification

On macOS (po4a 0.74 from Homebrew):

cmake -B build -DBUILD_MONOLITHIC=YES -DBUILD_REMOTEGUI=YES
cmake --build build --target po4a-update
# [100%] Refreshing translated manpages via po4a
# [100%] Built target po4a-update

The target invokes po4a po4a.config with the working directory set to docs/man/, matching what a translator would run by hand. When the committed .po and English .1 files are in mutual equilibrium (current state), po4a is a no-op. When they aren't (e.g., a translator updates a .po, or someone edits an English master), the target rewrites manpages.pot, syncs each manpages-LANG.po, and regenerates the corresponding *.LANG.1 files — and the translator commits the result.

Test plan

• Configure with po4a installed — target appears, cmake --build build --target po4a-update succeeds.
• Configure with po4a absent — no error, target simply doesn't exist (verified by reading the generated build/docs/man/Makefile).
• Confirmed the restored po4a.config matches the pre-cleanup: retire autotools, debian/, and dead platform/subtool trees #466 file with only the xas line removed (git show 337e236fb^:docs/man/po4a.config diff).