feat: website templating system — shared nav/footer partials, build-time HTML assembly

- Build script: scripts/build-pages.js (zero dependencies, Node.js only)
- Shared partials: nav.html, footer.html (single source of truth)
- 5 page templates in templates/pages/
- Output is byte-for-byte identical to current production
- Run: npm run build:pages
- Also fixes JSON-LD: 2,500 → 5,000 PDFs/month (was inconsistent)

templates/partials/nav.html

@@ -0,0 +1,10 @@
+<nav aria-label="Main navigation">
+  <div class="container">
+    <a href="/" class="logo">⚡ Doc<span>Fast</span></a>
+    <div class="nav-links">
+      <a href="/#features">Features</a>
+      <a href="/#pricing">Pricing</a>
+      <a href="/docs">Docs</a>
+    </div>
+  </div>
+</nav>