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

docs/index.md

@@ -15,4 +15,7 @@ Start with [Installation](setup/installation.md) to set up the toolchain and the
 - [Setup](setup/introduction.md): what SummerCMS is, installation, configuration and the move from WinterCMS.
 - [Architecture](architecture/introduction.md): the single binary, Go modules, the application lifecycle and the request lifecycle.
 - [Plugins](plugins/registration.md): registering a plugin, scheduling, extending other plugins and testing.
+- [Backend](backend/admin-controllers.md): admin controllers, forms, lists, relations, users, settings and the admin SPA.
+- [Database](database/models.md): models, migrations, queries, relations, casts, attachments and transactions.
+- [Services](services/configuration.md): configuration, events, routing, authentication, mail, jobs, realtime, search and the other services, plus what SummerCMS does not provide for the frontend.
 - [Console](console/introduction.md): the `summer` tool, the application binary's commands and writing your own.