Add GH pages workflow

Author: kinow

.github/workflows/gh-pages.yaml

@@ -0,0 +1,48 @@
+name: GitHub Pages sphinx deployment
+
+on:
+  push:
+    branches:
+      - main
+      - now-with-gh-pages
+  workflow_dispatch:
+
+jobs:
+  # Ref: https://stackoverflow.com/questions/57989790/using-github-actions-to-publish-documentation
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.9'
+
+      - name: Upgrade pip
+        run: |
+          # pip >= 20.1 so we can use ``pip cache dir``
+          python3 -m pip install --upgrade pip
+      - name: Get pip cache dir
+        id: pip-cache
+        run: echo "::set-output name=dir::$(pip cache dir)"
+
+      - name: Cache dependencies
+        uses: actions/cache@v3
+        with:
+          path: ${{ steps.pip-cache.outputs.dir }}
+          key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+          restore-keys: |
+            ${{ runner.os }}-pip-
+      - name: Install dependencies
+        run: python3 -m pip install -U -r ./requirements.txt
+
+      - name: Build documentation
+        run: make html
+
+      - name: Deploy
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./_build/html/
+          publish_branch: gh-pages-2