feat: documentation site

Docusaurus 3 docs-only site at docsite/, served statically by nginx under
/docs on the marketing host. Covers getting started (self-hosted install
through first server and first licence), the control plane, Vantage HQ,
a reference section and operations.

Wired into docker-compose.site.yml as docsite (3005:80) and into the
image build workflow, rebuilding on its own directory only. Never added
to the self-hosted compose file.

deploy/docker-compose.site.yml

@@ -55,4 +55,13 @@ services:
             - 3004:3000
         depends_on:
             - admin
+    # Static docs, served by nginx at vantage.hostxtra.co.uk/docs through its own
+    # proxy location. That location must sort ABOVE the catch-all forwarding to
+    # site:3003, or Next serves its own 404 for /docs. The container serves from
+    # /usr/share/nginx/html/docs because the proxy forwards the full path.
+    docsite:
+        image: gitea.hostxtra.co.uk/mrhid6/vantage/docsite:latest
+        restart: unless-stopped
+        ports:
+            - 3005:80
 networks: {}