feat(11.1-01): add summer docs:build with the docsite generator core

- internal/docsite loads docs/ with strict site.yaml and frontmatter decoding
- goldmark GFM pipeline renders pages into an embedded html/template shell
- emits .html pages, .md siblings, llms.txt, llms-full.txt, search-index.json
- output guard refuses unmarked non-empty dirs and --out inside --src or root
- docs/index.md and docs/setup/installation.md; /site/ is gitignored

internal/docsite/theme/templates/page.html

@@ -0,0 +1,32 @@
+<!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}}">
+</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>
+{{- end}}
+</ul>
+</div>
+{{- end}}
+</nav>
+<main id="content">
+<h1>{{.Title}}</h1>
+<p class="lead">{{.Description}}</p>
+{{.Content}}
+</main>
+</div>
+</body>
+</html>