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/pager.html

@@ -0,0 +1,14 @@
+{{define "pager" -}}
+{{- if or .Prev .Next}}
+<nav class="pager" aria-label="Previous and next page">
+{{- if .Prev}}
+<a class="pager-card pager-prev" href="{{.Prev.URL}}"><span class="pager-label">{{template "icon-chevron-left"}}Previous</span><span class="pager-title">{{.Prev.Title}}</span>{{if .Prev.Section}}<span class="pager-section">{{.Prev.Section}}</span>{{end}}</a>
+{{- else}}
+<span class="pager-empty"></span>
+{{- end}}
+{{- if .Next}}
+<a class="pager-card pager-next" href="{{.Next.URL}}"><span class="pager-label">Next{{template "icon-chevron-right"}}</span><span class="pager-title">{{.Next.Title}}</span>{{if .Next.Section}}<span class="pager-section">{{.Next.Section}}</span>{{end}}</a>
+{{- end}}
+</nav>
+{{- end}}
+{{- end}}