The entity is a fundamental part of the Entity Component System. Everything in your application that has data or an
identity of its own is an entity. However, an entity doesn't contain either data or behavior itself. Instead,
the data is stored in the components and the behavior is provided by the systems that process those
components. The entity acts as an identifier or key to the data stored in components.
The EntityManager class manages entities and they exist within a World. An
Entity struct refers to an entity, but isn't a reference. Rather, the Entity struct contains an
Entity.Index that you can use to access entity data, and a Entity.Version that you can
use to check whether the Index is still valid. Note that you must pass the Index or Version values to
relevant API methods, rather than accessing them directly.
To add or remove components, access components, or to destroy the entity, pass an Entity struct to methods of
the EntityManager, the EntityCommandBuffer, or the ComponentSystemBase.