refactor: stand up vantage-docs as its own repository

docsite/ becomes the repository root; the whole repository is the site.
The image is vantage/vantage-docs:latest, was mrhid6/vantage/docsite.

The workflow passes no build args, as the monorepo's did not: DOCS_URL,
DOCS_BASE_URL, APP_URL and HQ_URL have correct defaults in the
Dockerfile, and passing an unset repo variable would blank one rather
than leave it alone.

deploy/docker-compose.yml

@@ -0,0 +1,24 @@
+# The user documentation, served at vantage.hostxtra.co.uk/docs.
+#
+# A fragment, composed on top of the vantage repository's own file — see the
+# vantage-site fragment for the full command.
+#
+# A path on the marketing host, not a subdomain: *.vantage.hostxtra.co.uk is
+# the per-tenant instance namespace, and the control plane's APP_ROOT_LABEL
+# would read a `docs.` label as a tenant slug.
+#
+# The proxy location that routes here must sort ABOVE the catch-all forwarding
+# to site:3003, or Next answers the 404. It forwards the FULL path — it does
+# not strip /docs — so the proxy location, DOCS_BASE_URL and the directory the
+# image serves from (/usr/share/nginx/html/docs) must all agree. When they do
+# not, the HTML loads and every asset 404s.
+#
+# Unlike the Next apps this is a static build behind nginx, so it listens on 80
+# rather than 3000.
+services:
+    docsite:
+        image: gitea.hostxtra.co.uk/vantage/vantage-docs:latest
+        restart: unless-stopped
+        ports:
+            - 3005:80
+networks: {}