Clarify relationship to core commands and deprecate duplicate features

Problem/Motivation

Core has had a content:export command since 11.3.0, on top of the ability to import content with recipes which has been in since 10.x

It would be good to clarify what default_content does that's not handled by the core commands on the project page, deprecating anything that's now covered by core.

I don't currently use default_content for anything - this issue via a colleague who hadn't realised the export command got into core and noted there wasn't a mention on the project page.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes