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.

docs/setup/installation.md

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+---
+title: Installation
+description: Install the Go toolchain, PostgreSQL and the summer CLI you need to build a SummerCMS application.
+section: setup
+order: 20
+---
+# Installation
+
+SummerCMS is a Go module. An application requires it, lists its plugins in a `summer.yaml` manifest and builds everything into one binary with the `summer` CLI.
+
+## Requirements
+
+- Go 1.27.
+- PostgreSQL 16 for any application that uses the data layer. The database's default locale must be the ICU `pl-PL` locale, which lagoon checks when it connects.
+- Docker, only for the integration tests that start PostgreSQL or Mailpit containers.
+
+You do not need Node.js to build an application or these docs. It is needed only when you work on the admin SPA itself.
+
+## Install the summer CLI
+
+Clone the framework repository, then install the `summer` tool from its root:
+
+```sh
+go install ./cmd/summer
+summer --help
+```
+
+The tool builds and watches applications, scaffolds plugins, models, migrations and admin controllers, and builds this documentation. Check that the framework compiles and its unit tests pass:
+
+```sh
+go vet ./...
+go test -short ./...
+```
+
+`go test -short` skips the tests that need Docker. Run `go test ./...` without `-short` when Docker is available.

docs/site.yaml

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+# 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