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/astro.config.mjs

@@ -0,0 +1,26 @@
+// @ts-check
+import { defineConfig } from 'astro/config';
+import starlight from '@astrojs/starlight';
+
+// https://astro.build/config
+export default defineConfig({
+	integrations: [
+		starlight({
+			title: 'My Docs',
+			social: [{ icon: 'github', label: 'GitHub', href: 'https://github.com/withastro/starlight' }],
+			sidebar: [
+				{
+					label: 'Guides',
+					items: [
+						// Each item here is one entry in the navigation menu.
+						{ label: 'Example Guide', slug: 'guides/example' },
+					],
+				},
+				{
+					label: 'Reference',
+					items: [{ autogenerate: { directory: 'reference' } }],
+				},
+			],
+		}),
+	],
+});