feat(11.1-04): add the Backend section, the remaining Services pages and the concept map links

- docs/backend: admin controllers, forms, lists and filters, relation
  manager, users and permissions, settings, partials and widgets, admin SPA
- docs/services: storage, outbound HTTP, realtime, Web Push, search, parity
  testing and the Frontend and AJAX (not provided) page
- Examples for cabana (with testdata/docs YAML), fetchguard, lighthouse and
  its centrifugo driver, flare, beachcomber and typesense, tide; lighthouse
  and beachcomber TestDocs* regions run on their Postgres harnesses
- concept map rows link their guide pages and the not-provided rows the
  Frontend and AJAX page; index lists Backend, Database and Services
- TestDocsRequiredPages asserts the D-08 section order

modules/fetchguard/example_test.go

@@ -0,0 +1,52 @@
+package fetchguard_test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"time"
+
+	"git.golem15.com/golem15/summercms/modules/fetchguard"
+)
+
+func ExampleFetch() {
+	ctx := context.Background()
+	// Only the application's image host, at most 5 MiB within 5 seconds.
+	images := fetchguard.Policy{
+		Mode:       fetchguard.AllowHostsMode,
+		AllowHosts: []string{"images.example.com"},
+		MaxBytes:   5 << 20,
+		Timeout:    5 * time.Second,
+	}
+	// Any public host, for a URL a user pasted.
+	public := fetchguard.Policy{Mode: fetchguard.PublicOnlyMode}
+
+	for _, c := range []struct {
+		url    string
+		policy fetchguard.Policy
+	}{
+		{"http://images.example.com/cover.jpg", images},
+		{"https://cdn.attacker.example/cover.jpg", images},
+		{"https://127.0.0.1/admin", public},
+		{"https://169.254.169.254/latest/meta-data/", public},
+		{"https://[::ffff:10.0.0.1]/", public},
+		{"https://%zz", public},
+	} {
+		// The last argument is the application's config (app.Config), for
+		// limits the policy leaves at zero; nil uses the framework defaults.
+		_, err := fetchguard.Fetch(ctx, c.url, c.policy, nil)
+		var fe *fetchguard.Error
+		if errors.As(err, &fe) {
+			fmt.Println(fe.Reason, c.url)
+		}
+	}
+	fmt.Println(fetchguard.Defaults())
+	// Output:
+	// scheme http://images.example.com/cover.jpg
+	// invalid_url https://cdn.attacker.example/cover.jpg
+	// private_ip https://127.0.0.1/admin
+	// private_ip https://169.254.169.254/latest/meta-data/
+	// private_ip https://[::ffff:10.0.0.1]/
+	// invalid_url https://%zz
+	// 10485760 10s
+}