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

docs/index.md

@@ -0,0 +1,11 @@
+---
+title: SummerCMS documentation
+description: SummerCMS is a content management framework for Go, inspired by WinterCMS.
+section: index
+order: 0
+---
+# SummerCMS documentation
+
+SummerCMS keeps what makes WinterCMS productive (plugins that extend each other, YAML-driven admin forms and lists, console scaffolding) and compiles an application into a single Go binary. The framework modules, the application's plugins, the embedded admin SPA and the console commands all ship as one executable.
+
+Start with [Installation](setup/installation.md) to set up the toolchain and the `summer` CLI. The API reference section has one page per framework module.