add github pages deployment workflow

.github/workflows/deploy-docs.yml

@@ -0,0 +1,24 @@
+name: Deploy docs
+on: [push, pull_request]
+jobs:
+  build:
+    name: Deploy docs
+    runs-on: ubuntu-latest
+    steps:
+      - name: Download source
+        uses: actions/checkout@v3
+      - name: Install Crystal
+        uses: oprypin/install-crystal@v1
+      - name: Install Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.x'
+      - name: Install Python libs
+        run: pip install .
+      - name: Install Crystal libs
+        run: shards install
+      - name: Build site
+        run: mkdocs build
+      - name: Deploy to gh-pages
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+        run: mkdocs gh-deploy --force