feat(site): add section and heading anchors for deep-linking

Landing sections now carry ids (status, features, faq, collab, …) so any
section is directly linkable (e.g. /#faq). About and Legal headings get a
discreet, hover-revealed permalink via a goldmark heading render hook.
A scroll-margin-top offset keeps anchored targets clear of the sticky header.

site/layouts/_default/_markup/render-heading.html

@@ -0,0 +1,4 @@
+<h{{ .Level }} id="{{ .Anchor }}">
+  {{ .Text | safeHTML }}
+  <a class="heading-anchor" href="#{{ .Anchor }}" aria-label="Permalink">#</a>
+</h{{ .Level }}>