add crystal docs workflow

.github/workflows/documentation.yml

@@ -0,0 +1,29 @@
+ name: website
+
+on:
+  push:
+    branches: [ master ]
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Install Crystal
+        uses: crystal-lang/install-crystal@v1
+        with:
+          crystal: nightly
+      - name: Check out repository code
+        uses: actions/checkout@v2
+      - name: Install dependencies
+        run: shards install
+      - name: Generate documentation
+        run: crystal docs
+      -
+        name: Deploy to GitHub Pages
+        if: success()
+        uses: crazy-max/ghaction-github-pages@v2
+        with:
+          target_branch: gh-pages
+          build_dir: docs
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}