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/.gitignore

@@ -0,0 +1,21 @@
+# build output
+dist/
+# generated types
+.astro/
+
+# dependencies
+node_modules/
+
+# logs
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+
+
+# environment variables
+.env
+.env.production
+
+# macOS-specific files
+.DS_Store