ci: deploy the manual to GitHub Pages

.github/workflows/github_pages.yml

@@ -0,0 +1,28 @@
+name: GitHub Pages
+on:
+  push:
+    branches:
+      - master
+jobs:
+  publish:
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - uses: actions/checkout@v2
+    - uses: cachix/install-nix-action@v10
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: cachix/cachix-action@v6
+      with:
+        name: nix-community
+        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+    - run: |
+        nix-build -A docs.html
+        cp -r result/share/doc/home-manager public
+    - name: Deploy
+      uses: peaceiris/actions-gh-pages@v3
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        publish_dir: ./public