refactor(docs): split grouped API reference pages into per-procedure MDX files

Replace the single MDX file per API tag (e.g. `admin.mdx`, `titles.mdx`) with individual files for each procedure (e.g. `admin/admin.backups.create.mdx`). Each file renders a single `<APIPage>` with one operation, includes an inline description, and carries its own `_openapi` frontmatter.

Update `generate-api-docs.ts` to emit this per-procedure structure and adjust `source.ts` and `next.config.mjs` to resolve the new paths correctly.

docs/src/lib/source.ts

@@ -1,12 +1,13 @@
 import { docs } from "collections/server";
 import { type InferPageType, loader } from "fumadocs-core/source";
 import { lucideIconsPlugin } from "fumadocs-core/source/lucide-icons";
+import { openapiPlugin } from "fumadocs-openapi/server";
 
 // See https://fumadocs.dev/docs/headless/source-api for more info
 export const source = loader({
   baseUrl: "/docs",
   source: docs.toFumadocsSource(),
-  plugins: [lucideIconsPlugin()],
+  plugins: [lucideIconsPlugin(), openapiPlugin()],
 });
 
 export function getPageImage(page: InferPageType<typeof source>) {