build: add CI for publishing site to branch

.github/workflows/publish.yml

@@ -0,0 +1,27 @@
+name: Publish
+on:
+  push:
+    branches: [main]
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install
+      run: |
+        curl -LSfs https://raw.githubusercontent.com/crate-ci/gh-install/master/v1/install.sh | \
+          sh -s -- \
+          --git cobalt-org/cobalt.rs \
+          --crate cobalt \
+          --force \
+          --target x86_64-unknown-linux-gnu \
+          --tag v0.19.0
+        echo COBALT_BIN=~/.cargo/bin/cobalt >> $GITHUB_ENV
+    - name: Build
+      run: ${{ env.COBALT_BIN }} build
+    - name: Deploy
+      uses: peaceiris/actions-gh-pages@v3
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        publish_branch: site
+        publish_dir: _site