Write much better tutorials for the repo.

[jbloomAus]

I've explained how to use the repo in the docs, but this could be way better.

Some examples of pages what might be useful to add to the sphinx docs.

1. A page explaining what Minigrid is, and where to look at each MiniGrid environment (maybe include a table of environments by row with columns for different models (dt, bc, and ppo-traj, ppo standard).
2. A page explaining the difference between the online method (PPO) and offline methods, explaining why we need the trajectories for the latter.
3. A page/diagram explaining how trajectories are generated with the ppo method used to train offline agents.
4. MOST IMPORTANT: pages for stuff you think I won't have thought to explain since I wrote the repo.