feat(11.1-03): add the Architecture and Plugins docs sections

- architecture: introduction, Go modules and workspaces, application
  lifecycle, request lifecycle
- plugins: registration, scheduling, extending, testing
- verified Examples for backpack services, towel request context,
  pact schedules and festival events
- TestDocsRequiredPages lists the eight new pages

cmd/summer/docs_test.go

@@ -78,6 +78,14 @@ var requiredPages = []string{
 	"index",
 	"setup/installation",
 	"setup/coming-from-wintercms",
+	"architecture/introduction",
+	"architecture/go-modules-and-workspaces",
+	"architecture/application-lifecycle",
+	"architecture/request-lifecycle",
+	"plugins/registration",
+	"plugins/scheduling",
+	"plugins/extending",
+	"plugins/testing",
 }
 
 // TestDocsRequiredPages asserts that every required page is in the loaded