Add GitHub workflow to publish to Pages

It seems we can only enable GitHub Pages with subdirectories at the
root of the project. Ours is in ./site/docs so I think we need to
use an example like Hexo's to publish with a GitHub workflow.

See: https://hexo.io/docs/github-pages

.github/workflows/pages.yml

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