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/scripts/generate-api-docs.ts

@@ -4,6 +4,7 @@ import { openapi } from "../src/lib/openapi";
 void generateFiles({
   input: openapi,
   output: "./content/docs/api",
-  per: "tag",
+  groupBy: "tag",
   addGeneratedComment: true,
+  includeDescription: true,
 });