CI: don't build docs in parallel

pandas-dev#39366 showed that it made the
performance worse.

Author: afeld

.github/workflows/ci.yml

@@ -116,7 +116,7 @@ jobs:
     - name: Build documentation
       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 1 | 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)