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/package.json

@@ -0,0 +1,44 @@
+{
+    "name": "vantage-docs",
+    "version": "0.1.0",
+    "private": true,
+    "scripts": {
+        "start": "docusaurus start",
+        "build": "docusaurus build",
+        "serve": "docusaurus serve",
+        "clear": "docusaurus clear",
+        "typecheck": "tsc"
+    },
+    "dependencies": {
+        "@docusaurus/core": "^3.10.2",
+        "@docusaurus/preset-classic": "^3.10.2",
+        "@docusaurus/theme-mermaid": "^3.10.2",
+        "@easyops-cn/docusaurus-search-local": "^0.52.1",
+        "@mdx-js/react": "^3.0.0",
+        "clsx": "^2.1.1",
+        "prism-react-renderer": "^2.3.0",
+        "react": "^19.0.0",
+        "react-dom": "^19.0.0"
+    },
+    "devDependencies": {
+        "@docusaurus/module-type-aliases": "^3.10.2",
+        "@docusaurus/tsconfig": "^3.10.2",
+        "@docusaurus/types": "^3.10.2",
+        "typescript": "~5.6.2"
+    },
+    "browserslist": {
+        "production": [
+            ">0.5%",
+            "not dead",
+            "not op_mini all"
+        ],
+        "development": [
+            "last 3 chrome version",
+            "last 3 firefox version",
+            "last 5 safari version"
+        ]
+    },
+    "engines": {
+        "node": ">=20.0"
+    }
+}