feat(docs-site): scaffold Astro Starlight site

Scaffold docs/site/ via `npm create astro@latest ... --template
starlight`. Extend the docs collection schema in
src/content.config.ts with the four custom frontmatter fields (man,
site, manTitle, helpKeywords) needed by the generator in a later
task, using z.strictObject so unrecognized keys fail the build
instead of being silently stripped by Zod's default behavior.

Ignore generated site output (node_modules, dist, .astro, generated
content, and sidebar.json) in .gitignore.

docs/site/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "site",
+  "type": "module",
+  "version": "0.0.1",
+  "scripts": {
+    "dev": "astro dev",
+    "start": "astro dev",
+    "build": "astro build",
+    "preview": "astro preview",
+    "astro": "astro"
+  },
+  "dependencies": {
+    "@astrojs/starlight": "^0.41.4",
+    "astro": "^7.0.2",
+    "sharp": "^0.34.5"
+  }
+}