Merge pull request matplotlib#20718 from QuLogic/circle-parallel

Circle: Build docs in parallel.

Author: tacaswell

.circleci/config.yml

@@ -141,7 +141,7 @@ commands:
                [ "$CIRCLE_PR_NUMBER" = "" ]; then
               export RELEASE_TAG='-t release'
             fi
-            make html O="-T $RELEASE_TAG"
+            make html O="-T $RELEASE_TAG -j4"
             rm -r build/html/_sources
           working_directory: doc
       - save_cache:
@@ -167,6 +167,7 @@ jobs:
   docs-python38:
     docker:
       - image: cimg/python:3.8
+    resource_class: large
     steps:
       - checkout
       - check-skip