[CI] build docs faster (#10115)

I assume the CI machine should have at least 4 cores, so let's build docs faster

.circleci/config.yml

@@ -306,7 +306,7 @@ jobs:
                   key: v0.4-build_doc-{{ checksum "setup.py" }}
                   paths:
                       - '~/.cache/pip'
-            - run: cd docs && make html SPHINXOPTS="-W"
+            - run: cd docs && make html SPHINXOPTS="-W -j 4"
             - store_artifacts:
                 path: ./docs/_build