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

docs/examples/blog/config/config.yaml

@@ -0,0 +1,3 @@
+# Defaults for acme.blog, merged under the plugin ID: the application reads
+# this value as acme.blog.per_page and can override it in its own config.
+per_page: 15