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/next.config.mjs

@@ -14,7 +14,7 @@ const config = {
     return [
       {
         source: "/docs/api",
-        destination: "/docs/api/account",
+        destination: "/docs/api/account/account.removeAvatar",
         permanent: false,
       },
     ];