Add GitHub Pages deployment workflow

Deploys the Hugo docs site on push to main. Uses the same mise-action
pattern as PR #221. All actions are SHA-pinned.

After merge, the Pages source needs to flip from "Deploy from a branch"
(gh-pages) to "GitHub Actions" — handled in the mm_website Terraform PR.
The legacy gh-pages branch can then be deleted.

For STF-448.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: oschwald

.github/workflows/pages.yml

@@ -0,0 +1,54 @@
+name: Deploy to GitHub Pages
+
+on:
+  push:
+    branches: ["main"]
+  workflow_dispatch:
+
+permissions: {}
+
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+
+      - name: Setup mise
+        uses: jdx/mise-action@6d1e696aa24c1aa1bcc1adea0212707c71ab78a8 # v3.6.1
+        with:
+          cache: true
+
+      - name: Build docs
+        env:
+          HUGO_BASEURL: https://${{ github.repository_owner }}.github.io/${{ github.event.repository.name }}/
+        run: mise run build-docs
+
+      - name: Configure Pages
+        uses: actions/configure-pages@983d7736d9b0ae728b81ab479565c72886d7745b # v5.0.0
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@7b1f4a764d45c48632c6b24a0339c27f5614fb0b # v4.0.0
+        with:
+          path: docs/public
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    permissions:
+      pages: write
+      id-token: write
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@d6db90164ac5ed86f2b6aed7e0febac5b3c0c03e # v4.0.5