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/setup/installation.md

@@ -0,0 +1,35 @@
+---
+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.