feat: write the site for Starlight behind a flag

generate_site.py --engine starlight writes the pages, navigation and
data files of an Astro Starlight project in website/. Links resolve to
site addresses, every heading keeps its MkDocs id, and pages split by
size are measured in the MkDocs Markdown so both builds cut the same
files. The 839 pages and their 45,613 ids match the MkDocs build.

website/src/content.config.ts

@@ -0,0 +1,22 @@
+import { defineCollection } from 'astro:content';
+import { z } from 'astro/zod';
+import { docsLoader } from '@astrojs/starlight/loaders';
+import { docsSchema } from '@astrojs/starlight/schema';
+
+// The generator's own front matter, typed: a page with a misspelt level fails the build.
+export const collections = {
+	docs: defineCollection({
+		loader: docsLoader(),
+		schema: docsSchema({
+			extend: z.object({
+				// Who the page is written for, from someone who knows nothing
+				// (start) to someone adding to the project (contribute).
+				level: z.enum(['start', 'setup', 'explore', 'reference', 'contribute']).optional(),
+				// Search ranking of the first two levels.
+				boost: z.number().min(1).max(10).optional(),
+				// The date the page's content carries, for the sitemap.
+				lastmod: z.string().optional(),
+			}),
+		}),
+	}),
+};