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/logo.svg

@@ -0,0 +1,25 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 130 130">
+  <rect width="130" height="130" rx="24" fill="#faf8f5"/>
+  <circle cx="65" cy="14" r="5" fill="#5b8c5a"/>
+  <circle cx="58" cy="30" r="6" fill="#7ab87a"/>
+  <circle cx="74" cy="30" r="4" fill="#6ba86b"/>
+  <circle cx="51.5" cy="46" r="6.5" fill="#5b8c5a"/>
+  <circle cx="65" cy="46" r="5" fill="#7ab87a"/>
+  <circle cx="81" cy="46" r="4" fill="#6ba86b"/>
+  <circle cx="44" cy="62" r="7" fill="#6ba86b"/>
+  <circle cx="61" cy="62" r="5.5" fill="#5b8c5a"/>
+  <circle cx="76" cy="62" r="4.5" fill="#7ab87a"/>
+  <circle cx="89.5" cy="62" r="3.5" fill="#6ba86b"/>
+  <circle cx="36.5" cy="78" r="7.5" fill="#5b8c5a"/>
+  <circle cx="54" cy="78" r="6" fill="#7ab87a"/>
+  <circle cx="70" cy="78" r="5" fill="#6ba86b"/>
+  <circle cx="84" cy="78" r="4" fill="#5b8c5a"/>
+  <circle cx="97.5" cy="78" r="3.5" fill="#7ab87a"/>
+  <circle cx="44" cy="94" r="6.5" fill="#6ba86b"/>
+  <circle cx="61" cy="94" r="5.5" fill="#5b8c5a"/>
+  <circle cx="76" cy="94" r="4.5" fill="#7ab87a"/>
+  <circle cx="89.5" cy="94" r="3.5" fill="#6ba86b"/>
+  <circle cx="51.5" cy="110" r="5.5" fill="#5b8c5a"/>
+  <circle cx="65" cy="110" r="4.5" fill="#7ab87a"/>
+  <circle cx="81" cy="110" r="3.5" fill="#6ba86b"/>
+</svg>