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

docs/site.yaml

@@ -13,5 +13,9 @@ llms_notes:
 sections:
   - name: setup
     title: Setup
+  - name: architecture
+    title: Architecture
+  - name: plugins
+    title: Plugins
   - name: api
     title: API reference