Add a deployment step to gh-pages branch

Following the instructions from CircleCI's blog
https://circleci.com/blog/deploying-documentation-to-github-pages-with-continuous-integration/

Author: QuLogic

.circleci/config.yml

@@ -21,7 +21,37 @@ jobs:
       - store_artifacts:
           path: docs/_build/html/
 
+  deploy:
+    docker:
+      - image: "node:8.10.0"
+    steps:
+      - attach_workspace:
+          at: html
+      - run:
+          name: Install and configure dependencies
+          command: |
+            npm install -g --silent gh-pages@2.0.1
+            git config user.email "MatplotlibCircleBot@nomail"
+            git config user.name "MatplotlibCircleBot"
+      - add_ssh_keys:
+          fingerprints:
+            - "25:2e:77:93:7e:0e:af:77:b0:be:e7:fe:86:2c:5b:96"
+      - run:
+          name: "Deploy new docs"
+          command: |
+            touch html/.nojekyll  # Disable jekyll builds on GitHub.
+            gh-pages \
+              --dotfiles \
+              --message "[skip ci] Doc build of $CIRCLE_SHA1" \
+              --dist html
+
 workflows:
   main:
     jobs:
       - build_docs
+      - deploy:
+          requires:
+            - build_docs
+          filters:
+            branches:
+              only: main