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/src/content.config.ts

@@ -0,0 +1,25 @@
+import { defineCollection } from 'astro:content';
+import { z } from 'astro/zod';
+import { docsLoader } from '@astrojs/starlight/loaders';
+import { docsSchema } from '@astrojs/starlight/schema';
+
+export const collections = {
+  docs: defineCollection({
+    loader: docsLoader(),
+    schema: docsSchema({
+      // z.strictObject (not z.object) so that Zod v4's default key-stripping
+      // doesn't silently swallow typos in generated frontmatter — an unknown
+      // key like `bogusField` must fail the build, not disappear quietly.
+      extend: z.strictObject({
+        // Include this page in the generated man page.
+        man: z.boolean().default(true),
+        // Publish this page to the website.
+        site: z.boolean().default(true),
+        // Verbatim heading text used by the man page renderer.
+        manTitle: z.string().optional(),
+        // Keywords resolving to this page via `help config <keyword>`.
+        helpKeywords: z.array(z.string()).default([]),
+      }),
+    }),
+  }),
+};