Replace the lesson resources with a data format easier to edit #1188
Description
Currently, all the lesson/course content is stored as Godot resources. We used that at the time to rely on the built-in serialization and loading in the engine, but it gets in the way when it comes to updating content.
This task is about replacing these resources with a more editable format to make maintenance and adding content much easier moving forward.
We use the BBCode formatting of Godot already for the front end and it's a very straightforward format to parse. So I think we'll use BBCode for everything and as a migration path, we'll parse the BBCode to the resources currently used by the application at runtime.
This will also simplify the translation extraction and make it easy to address issues like #835