feat(11.1-05): add the acme.blog walkthrough plugin with its model, migration and posts route

- docs/examples/blog: scaffolder output for acme.blog (make:plugin, make:model)
  in the root module, with a Post model, a fill allow-list and NewPost, the
  create migration and GET /api/blog/posts paginated through lagoon
- short tests activate the plugin, check the route with surf, the fill
  allow-list and the migration order
- docs/setup/porting-a-plugin.md: registration, model, migrations and routes
  sections with src= copies of the plugin
- TestDocsRequiredPages requires setup/porting-a-plugin

cmd/summer/docs_test.go

@@ -124,6 +124,7 @@ var requiredPages = []string{
 	"services/search",
 	"services/parity-testing",
 	"services/frontend-and-ajax",
+	"setup/porting-a-plugin",
 }
 
 // sectionOrder is the D-08 sidebar order: WinterCMS's documentation order,