ci: add deployment with github actions

.github/workflows/deploy.yml

@@ -0,0 +1,24 @@
+on:
+  push:
+    branches: [main]
+    paths:
+      - src/**
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          submodules: true
+      - name: Setup mdBook
+        uses: peaceiris/actions-mdbook@v2
+        with:
+          mdbook-version: 'latest'
+      - run: mdbook build
+      - name: Deploy
+        uses: peaceiris/actions-gh-pages@v3
+        if: ${{ github.ref == 'refs/heads/main' }}
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./book