site: bring the official website + Pages deploy workflow to main (#424)

The site.yml workflow deploys on push to main, so the site only goes live once
site/ is on main. Bring site/index.html + icons + the workflow so a GitHub
Pages deploy (Settings -> Pages -> Source: GitHub Actions) publishes it at
justvugg.github.io/colibri.

Refs #424

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/site.yml

@@ -0,0 +1,36 @@
+name: Deploy website
+
+# Publishes site/ to GitHub Pages. One-time repo setup:
+#   Settings → Pages → Build and deployment → Source: "GitHub Actions".
+# Custom domain later: add site/CNAME with the bare domain, point DNS
+# (A/AAAA to GitHub Pages IPs or CNAME to <org>.github.io), done.
+
+on:
+  push:
+    branches: [main]
+    paths: ['site/**', '.github/workflows/site.yml']
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/configure-pages@v5
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: site
+      - id: deployment
+        uses: actions/deploy-pages@v4