add docs build action

.github/workflows/deploy-docs.yml

@@ -0,0 +1,28 @@
+name: Deploy docs to GitHub Pages
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup Rust
+      uses: actions-rs/toolchain@v1
+      with:
+        profile: minimal
+        toolchain: stable
+        override: true
+    - name: Install mdBook
+      run: cargo install mdbook
+    - name: Build mdBook
+      run: mdbook build docs
+    - name: Deploy to GitHub Pages
+      uses: peaceiris/actions-gh-pages@v3
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        publish_dir: ./docs/book
+