docs: relocate website to docs/website/, add GitHub Actions deployment (#557)

Separates website files from non-website documentation:
- Move all website content (api/, articles/, styles/, fonts/, images/,
  index.html, etc.) to docs/website/
- Keep docs/issues/ and docs/plans/ at docs/ root
- Add .github/workflows/docs.yml for GitHub Actions Pages deployment
- Update docfx_project/docfx.json to output to docs/website/

The new workflow deploys from docs/website/ using actions/deploy-pages,
replacing the legacy "Deploy from a branch" approach which only supports
/ or /docs paths.

Post-merge: Change GitHub Pages source to "GitHub Actions" in repo settings.

Closes #556

Author: Nucs

.github/workflows/docs.yml

@@ -0,0 +1,39 @@
+name: Deploy Docs
+
+on:
+  push:
+    branches: ["master", "main"]
+    paths:
+      - 'docs/website/**'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
+
+jobs:
+  deploy:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Pages
+        uses: actions/configure-pages@v5
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: 'docs/website'
+
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4

docfx_project/docfx.json

@@ -49,7 +49,7 @@
         ]
       }
     ],
-    "dest": ".",
+    "dest": "../docs/website",
     "globalMetadataFiles": [],
     "fileMetadataFiles": [],
     "template": [