feat(11.1-04): add the Backend section, the remaining Services pages and the concept map links

- docs/backend: admin controllers, forms, lists and filters, relation
  manager, users and permissions, settings, partials and widgets, admin SPA
- docs/services: storage, outbound HTTP, realtime, Web Push, search, parity
  testing and the Frontend and AJAX (not provided) page
- Examples for cabana (with testdata/docs YAML), fetchguard, lighthouse and
  its centrifugo driver, flare, beachcomber and typesense, tide; lighthouse
  and beachcomber TestDocs* regions run on their Postgres harnesses
- concept map rows link their guide pages and the not-provided rows the
  Frontend and AJAX page; index lists Backend, Database and Services
- TestDocsRequiredPages asserts the D-08 section order

modules/beachcomber/export_docs_test.go

@@ -0,0 +1,47 @@
+package beachcomber
+
+import (
+	"testing"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/compass"
+	"git.golem15.com/golem15/summercms/modules/lagoon"
+	"gorm.io/gorm"
+)
+
+// DocsApp returns an app with the given config settings on a fresh migrated
+// database of this package's Postgres harness, with the database published,
+// for the database-backed docs examples in example_test.go. It skips under
+// -short and fails when the harness has no database, like the package's
+// other database tests.
+func DocsApp(t *testing.T, settings map[string]any) (*backpack.App, *gorm.DB) {
+	t.Helper()
+	db, dsn := migratedDB(t)
+	cfg, err := compass.Open(compass.Options{Dir: t.TempDir(), Env: "testing", Environ: []string{}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := cfg.Set("database.dsn", dsn); err != nil {
+		t.Fatal(err)
+	}
+	for k, v := range settings {
+		if err := cfg.Set(k, v); err != nil {
+			t.Fatal(err)
+		}
+	}
+	app := backpack.New(cfg)
+	gdb, err := lagoon.Use(t.Context(), db)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := lagoon.Publish(app, db, gdb); err != nil {
+		t.Fatal(err)
+	}
+	return app, gdb
+}
+
+// DocsUseEngine replaces the engine of svc, for docs examples whose toy
+// engine must not join the global engine registry.
+func DocsUseEngine(svc *Service, e Engine) {
+	svc.engine = e
+}