Add an API docs topic to cover entity characteristics in general and how they work

The entity system provides a range of interfaces for use with the entity class that add behaviour to an entity type.

Adding a topic on this:

- helps developers understand how entity types can be enhanced
- gives us something to group all the various interfaces so they can be more easily discovered by developers.

See #2980340: [meta] improve DX & documentation for entity type characteristics for details.