chore: import upstream snapshot with attribution

apps/docs/source.config.ts

@@ -0,0 +1,22 @@
+import { defineConfig, defineDocs, frontmatterSchema } from 'fumadocs-mdx/config'
+import { z } from 'zod'
+
+/**
+ * Diátaxis page type — an internal authoring taxonomy surfaced to readers as a
+ * small badge near the page title. Optional so existing pages render unchanged
+ * until backfilled. Only collections may be exported from this file, so the
+ * shared type lives in `@/lib/source` (`DocsPageType`).
+ */
+export const docs = defineDocs({
+  dir: 'content/docs',
+  docs: {
+    schema: frontmatterSchema.extend({
+      pageType: z.enum(['tutorial', 'guide', 'reference', 'concept']).optional(),
+    }),
+    postprocess: {
+      includeProcessedMarkdown: true,
+    },
+  },
+})
+
+export default defineConfig()