docs site: add MkDocs publishing layer (build, deploy, push-to-main webhook)

Renders the repository's Markdown (which stays where it is) into a
member-readable static site with Material for MkDocs: co-op brand colours
and logo, self-hosted OFL Figtree, offline search, no external hosts.

- publishing/build.sh   stage .site-src, render .site-build (mkdocs --strict)
- publishing/deploy.sh  write the build into the Surfer app container
- publishing/webhook-receiver.py + .service   Gitea push hook on main
                        triggers pull -> build -> deploy (agent host, no new
                        Cloudron app)

publishing/requirements.txt

@@ -0,0 +1,3 @@
+# Pinned build toolchain for the documentation site.
+# Reproducible: `pip install -r publishing/requirements.txt && mkdocs build`
+mkdocs-material==9.8.0