ci(docs): publish through the shared GitHub Pages workflow

[jimisola]

What & Why

publish_gh_pages.yml was a line-for-line copy of common-publish-to-github-pages.yml — same checkout, Pages config, Node setup, Antora build, artifact upload, deploy. Any fix to one had to be remembered in the other. It now calls the shared workflow.

Two differences, both in the shared one’s favour:

• It passes --log-failure-level=fatal to Antora, so a fatal is a failed build rather than a quietly broken site.
• The mkdir -p docs/build/site guard is gone. It made an Antora run that produced nothing deploy an empty directory instead of failing.

The repo’s own package-lock.json still pins Antora 3.1.15: the shared build action runs npm ci when it finds a lockfile, and falls back to a pinned install only when there is none.

Author checklist

• actionlint clean
• Conventional Commit title, DCO sign-off
• Triggers unchanged (workflow_dispatch, release: created, push to main)

Test Plan

Merge after reqstool/.github#66 — the shared workflow gains the build-antora-site action there.

Then push to main (or dispatch) and confirm the site builds and deploys as before.