DOC: parallelize the documentation build in CI

Author: afeld

.github/workflows/ci.yml

@@ -114,9 +114,11 @@ jobs:
         source activate pandas-dev
         python web/pandas_web.py web/pandas --target-path=web/build
     - name: Build documentation
+      # num-jobs corresponds to number of CPUs in runner
+      # https://docs.github.com/en/actions/reference/specifications-for-github-hosted-runners#supported-runners-and-hardware-resources
       run: |
         source activate pandas-dev
-        doc/make.py --warnings-are-errors | tee sphinx.log ; exit ${PIPESTATUS[0]}
+        doc/make.py --warnings-are-errors --num-jobs 2 | tee sphinx.log ; exit ${PIPESTATUS[0]}
 
     # This can be removed when the ipython directive fails when there are errors,
     # including the `tee sphinx.log` in te previous step (https://github.com/ipython/ipython/issues/11547)