added github action to copy docs to gh-pages branch

.github/workflows/gh-pages.yml

@@ -0,0 +1,18 @@
+name: github pages
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  deploy:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Deploy to gh-pages
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: "./docs"