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/sidebar.html

@@ -0,0 +1,16 @@
+{{define "sidebar" -}}
+<div class="sidebar-panel" id="sidebar">
+<nav class="sidebar" aria-label="Documentation">
+{{- range .Nav}}
+<div class="sidebar-section{{if .API}} sidebar-api{{end}}">
+<p class="sidebar-title">{{.Title}}</p>
+<ul>
+{{- range .Items}}
+<li><a href="{{.URL}}"{{if .Current}} aria-current="page"{{end}}>{{.Title}}</a></li>
+{{- end}}
+</ul>
+</div>
+{{- end}}
+</nav>
+</div>
+{{- end}}