Add workflow to publish website on github pages

.github/workflows/website.yml

@@ -0,0 +1,24 @@
+name: "Website"
+
+on:
+  push:
+    branches:
+    - master
+
+jobs:
+  code:
+    name: Website
+    runs-on: ubuntu-18.04
+    steps:
+      - name: Check out repository code
+        uses: actions/checkout@v2
+      - name: Install sphinx
+        run: sudo apt-get install python3-sphinx python3-sphinxcontrib.bibtex
+      - name: Build html documentation
+        run: cd doc && make html && cd -
+      - name: Deploy
+        uses: peaceiris/actions-gh-pages@v2.4.0
+        env:
+          ACTIONS_DEPLOY_KEY: ${{ secrets.ACTIONS_DEPLOY_KEY }}
+          PUBLISH_BRANCH: gh-pages
+          PUBLISH_DIR: ./doc/build/html/