feat(11.1-02): add the documentation theme, chroma highlighting and docs:serve

- WinterCMS-style shell: header with search and theme toggle, grouped
  sidebar, on-page TOC, pager, page actions, callouts, heading
  permalinks, footer and a 404 page
- fenced code highlighted at build time by chroma/v2 into tok-* classes,
  with a copy button; no inline script, style or handler
- vendored DM Sans/DM Mono fonts and Lucide icons with their licences
- client-side search over search-index.json built with textContent only
- summer docs:serve builds into a temp dir, serves on loopback by default,
  returns 404.html with status 404 and rebuilds on change

internal/docsite/theme/templates/404.html

@@ -0,0 +1,22 @@
+<!doctype html>
+<html lang="en">
+<head>
+{{template "head" .}}
+</head>
+<body class="docs docs-404">
+{{template "shell-start" .}}
+<main id="content" tabindex="-1">
+<div class="page">
+<article class="page-body">
+<h1>{{.Title}}</h1>
+<p class="lead">{{.Description}}</p>
+<div class="prose">
+<p>Use the sidebar or search to find what you need, or go back to the <a href="{{.HomeURL}}">documentation home</a>.</p>
+</div>
+</article>
+</div>
+{{template "footer" .}}
+</main>
+{{template "shell-end" .}}
+</body>
+</html>