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

modules/festival/example_test.go

@@ -0,0 +1,35 @@
+package festival_test
+
+import (
+	"context"
+	"fmt"
+
+	"git.golem15.com/golem15/summercms/modules/festival"
+)
+
+// PostPublished is the event the acme.blog plugin fires after a post goes live.
+type PostPublished struct {
+	Title string
+}
+
+func ExampleBus_Fire() {
+	bus := festival.New() // in a plugin, use app.Events
+
+	// acme.search and acme.notify extend acme.blog from their Boot steps.
+	bus.Listen("acme.search", func(ctx context.Context, e PostPublished) error {
+		fmt.Println("index", e.Title)
+		return nil
+	})
+	bus.ListenPriority("acme.notify", 10, func(ctx context.Context, e PostPublished) error {
+		fmt.Println("notify subscribers of", e.Title)
+		return nil
+	})
+
+	// acme.blog fires the event; higher priorities run first.
+	if err := bus.Fire(context.Background(), PostPublished{Title: "Hello"}); err != nil {
+		fmt.Println(err)
+	}
+	// Output:
+	// notify subscribers of Hello
+	// index Hello
+}