ci: install pnpm in docs release workflow

The docs release workflow runs `uv run poe build-docs`, which shells out
to `pnpm`, but pnpm was not installed on the runner — causing the job to
fail with `pnpm: not found`. Add the missing pnpm install step, matching
the pattern already used in `manual_version_docs.yaml`.

Author: vdusek

.github/workflows/manual_release_docs.yaml

@@ -55,6 +55,11 @@ jobs:
       - name: Install Python dependencies
         run: uv run poe install-dev
 
+      - name: Install pnpm and website dependencies
+        uses: apify/workflows/pnpm-install@main
+        with:
+          working-directory: website
+
       - name: Build Docusaurus docs
         run: uv run poe build-docs
         env: