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/backpack/example_test.go

@@ -0,0 +1,42 @@
+package backpack_test
+
+import (
+	"fmt"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/compass"
+)
+
+// Greeter is a service the acme.greeter plugin offers to other plugins.
+type Greeter interface {
+	Greet(name string) string
+}
+
+type englishGreeter struct{}
+
+func (englishGreeter) Greet(name string) string { return "Hello, " + name }
+
+func ExampleApp_Publish() {
+	app := backpack.New(&compass.Config{})
+	app.SetPlugins([]string{"acme.greeter", "acme.blog"})
+
+	// acme.greeter, in its Register step: publish under the interface type.
+	var greeter Greeter = englishGreeter{}
+	if err := app.Publish(greeter); err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	// acme.blog, in its Boot step: look the service up by the same type.
+	if app.HasPlugin("acme.greeter") {
+		if found, ok := app.Lookup[Greeter](); ok {
+			fmt.Println(found.Greet("blog"))
+		}
+	}
+
+	// A second Publish under the same type is refused.
+	fmt.Println(app.Publish(greeter) != nil)
+	// Output:
+	// Hello, blog
+	// true
+}