Document agent.status and env.done when finishing early

Story

As a TEAM MEMBER, I want clearly document the behavior of agent.status and env.done so that USERS of Flatland have a clear understanding of the meaning of these parameters.

Meaning of different parameters: env.done = TRUE The environment has terminated --> All agents are done or the max_timesteps was achieved agent.status=DONE --> Agent finished its tasks and remains in the environment agent.status=DONE_REMOVED --> Agent finishes its individual task and was removed from the environment.

Acceptance Criteria

• Documentation in the main documentation about the different cases done=True vs agent.status = DONE or DONE_REMOVE