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.

docsite/tsconfig.json

@@ -0,0 +1,7 @@
+{
+    "extends": "@docusaurus/tsconfig",
+    "compilerOptions": {
+        "baseUrl": "."
+    },
+    "exclude": [".docusaurus", "build"]
+}