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/page.html

@@ -1,32 +1,62 @@
+{{define "head" -}}
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<meta name="color-scheme" content="light dark">
+<title>{{.DocTitle}}</title>
+<meta name="description" content="{{.Description}}">
+<meta property="og:title" content="{{.Title}}">
+<meta property="og:description" content="{{.Description}}">
+{{- if .MarkdownURL}}
+<link rel="alternate" type="text/markdown" href="{{.MarkdownURL}}">
+{{- end}}
+<script src="{{.Assets}}/theme-init.js"></script>
+<link rel="stylesheet" href="{{.Assets}}/site.css">
+<script src="{{.Assets}}/site.js" defer></script>
+<script src="{{.Assets}}/search.js" defer></script>
+{{- end -}}
+{{define "shell-start" -}}
+<a class="skip-link" href="#content">Skip to content</a>
+{{template "header" .}}
+<div class="layout">
+{{template "sidebar" .}}
+<div class="scrim" hidden></div>
+{{- end -}}
+{{define "shell-end" -}}
+</div>
+{{template "search" .}}
+<div class="sr-only" id="live-region" aria-live="polite"></div>
+{{- end -}}
 <!doctype html>
 <html lang="en">
 <head>
-<meta charset="utf-8">
-<meta name="viewport" content="width=device-width, initial-scale=1">
-<title>{{.DocTitle}}</title>
-<meta name="description" content="{{.Description}}">
-<link rel="stylesheet" href="{{.CSS}}">
+{{template "head" .}}
 </head>
 <body class="docs">
-<div class="layout">
-<nav class="sidebar" aria-label="Documentation">
-<a class="sidebar-home" href="{{.HomeURL}}">SummerCMS</a>
-{{- range .Nav}}
-<div class="sidebar-section">
-<p class="sidebar-title">{{.Title}}</p>
-<ul>
-{{- range .Items}}
-<li><a href="{{.URL}}"{{if .Current}} aria-current="page"{{end}}>{{.Title}}</a></li>
+{{template "shell-start" .}}
+<main id="content" tabindex="-1">
+<div class="page{{if .TOC}} has-toc{{end}}">
+<article class="page-body">
+{{- if .Eyebrow}}
+<p class="eyebrow">{{.Eyebrow}}</p>
 {{- end}}
-</ul>
-</div>
-{{- end}}
-</nav>
-<main id="content">
 <h1>{{.Title}}</h1>
 <p class="lead">{{.Description}}</p>
-{{.Content}}
-</main>
+<div class="page-actions">
+{{- if .EditURL}}
+<a href="{{.EditURL}}">{{template "icon-pencil"}}<span>Edit this page</span></a>
+{{- end}}
+<a href="{{.MarkdownURL}}">{{template "icon-file-text"}}<span>View as Markdown</span></a>
 </div>
+{{template "toc-inline" .}}
+<div class="prose">
+{{.Content}}
+</div>
+{{template "pager" .}}
+</article>
+{{template "toc" .}}
+</div>
+{{template "footer" .}}
+</main>
+{{template "shell-end" .}}
 </body>
 </html>