doc: add new doc website

* use mmdoc
* add github pages action to auto publish
* do not edit README for now, will follow up with a commit directs
people to the doc site

.github/workflows/doc.yml

@@ -0,0 +1,36 @@
+name: doc
+
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: "pages"
+  cancel-in-progress: true
+
+jobs:
+  publish:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v20
+      - run: nix build .#doc
+      - name: Setup Pages
+        uses: actions/configure-pages@v3
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v1
+        with:
+          path: 'result/multi'
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v1