CI: Build the Java docs with the current docs assembly

[tony]

Summary

• Show the Ruby and Lua entries on libtmux.org's Java pages. This repository builds its tree with libtmux/docs's own assembly, pinned to a commit from before Ruby and Lua joined the site in libtmux/docs#18, so every push to master rebuilt the Java pages with the old menu. Both pins, the docs checkout and the reusable deploy workflow, move to the current libtmux/docs commit together, as the workflow's comment requires.

Compatibility

• The build call is unchanged. scripts/build-site.sh --ports java --skip-pagefind with LIBTMUX_DOCS_CHECKOUT_JAVA still works; the script's new source-bound mode switches on only when its own variables are set.
• The deploy inputs are unchanged. path-prefix, artifact, version-kind, port, version, is-default, environment and the three secrets all still exist in reusable-deploy.yml.

Test plan

• Built locally with the old pin, the Java landing page links neither Ruby nor Lua; with the new pin it links each three times
• The docs workflow publishes from master after merge