Docs: Document backend package hierarchy model (grafana#31930)

* Docs: Document backend package hierarchy model

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

contribute/architecture/backend/package-hierarchy.md

@@ -0,0 +1,16 @@
+# Package hierarchy
+
+The Go package hierarchy in Grafana should be organized logically (Ben Johnson's
+[article](https://medium.com/@benbjohnson/standard-package-layout-7cdbc8391fc1) served as inspiration), according to the 
+following principles:
+
+* Domain types and interfaces should be in "root" packages (not necessarily at the very top, of the hierarchy, but
+  logical roots)
+* Sub-packages should depend on roots - sub-packages here typically contain implementations, for example of services
+
+## Practical example
+
+The `pkg/plugins` package contains plugin domain types, for example `DataPlugin`, and also interfaces
+such as `RequestHandler`. Then you have the `pkg/plugins/managers` subpackage, which contains concrete implementations
+such as the service `PluginManager`. The subpackage `pkg/plugins/backendplugin/coreplugin` contains `plugins.DataPlugin`
+implementations.