feat(workflows): add workflow to publish docs site

- publish to gh-pages branch by default

.github/workflows/publish.yml

@@ -0,0 +1,27 @@
+name: Publish
+on:
+  push:
+    branches: [master]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set up Python 3.8
+        uses: actions/setup-python@v1
+        with:
+          python-version: 3.8
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+      - name: Build
+        run: |
+          mkdocs build -c -f mkdocs.yml
+      - name: Publish
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./site