[Admin] allow to reuse or back port custom backend menu items in solidus_admin

[elia]

Whenever a store defines additional menu items it should be possible to easily back port the customizations to SolidusAdmin.

Some documentation on how to do that should be provided.

Long term solidus_backend items should stop relying on partials and a mix of data and templates, and just data. That way we could use the data structure directly to generate also the SolidusAdmin menu and have a swift migration.