Actions: Add a workflow to deploy the zcashd book

.github/workflows/book.yml

@@ -0,0 +1,32 @@
+name: zcashd book
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup mdBook
+        uses: peaceiris/actions-mdbook@v1
+        with:
+          mdbook-version: '0.4.5'
+
+      - name: Install mdbook-katex
+        uses: actions-rs/cargo@v1
+        with:
+          command: install
+          args: mdbook-katex
+
+      - name: Build zcashd book
+        run: mdbook build book/
+
+      - name: Deploy to GitHub Pages
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./book/book