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/site.yaml

@@ -0,0 +1,15 @@
+# Documentation site configuration, read by `summer docs:build`.
+# Sections are listed in sidebar order; each must have at least one page.
+title: SummerCMS
+description: "SummerCMS is a content management framework for Go, inspired by WinterCMS."
+base_url: ""
+edit_url: "https://git.golem15.com/golem15/summercms/_edit/master/{path}"
+source_url: "https://git.golem15.com/golem15/summercms/src/branch/master/{path}"
+llms_notes:
+  - "Plugins are Go modules compiled into the application binary and registered at build time; nothing is loaded at runtime."
+  - "The data layer supports PostgreSQL only."
+  - "SummerCMS is headless: it serves a JSON API and an admin SPA, with no frontend themes."
+  - "It targets Go 1.27."
+sections:
+  - name: setup
+    title: Setup