CI: speedup docs build (#30574)

old-commit-hash: 5dba9187e5

.github/workflows/docs.yaml

@@ -32,7 +32,7 @@ jobs:
         ${{ env.RUN }} "scons -j$(nproc)"
     - name: Build docs
       run: |
-        ${{ env.RUN }} "apt update && apt install -y doxygen && cd docs && make html"
+        ${{ env.RUN }} "apt update && apt install -y doxygen && cd docs && make -j$(nproc) html"
 
     - uses: actions/checkout@v4
       if: github.ref == 'refs/heads/master' && github.repository == 'commaai/openpilot'