CI: Add workflow to render latest documentation

.github/workflows/book.yml

@@ -0,0 +1,33 @@
+name: librustzcash documentation
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  deploy:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
+
+      - name: Build latest rustdocs
+        uses: actions-rs/cargo@v1
+        with:
+          command: rustdoc
+          args: --workspace --all-features -- --cfg docsrs
+
+      - name: Move latest rustdocs into book
+        run: |
+          mkdir -p ./book/book/rustdoc
+          mv ./target/doc ./book/book/rustdoc/latest
+
+      - name: Deploy to GitHub Pages
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./book/book