Add a "How do I .... ?" section to the documentation

The goal of a How do I...? section in the documentation is to aggregate simple code snippets for a series of common tasks of different complexity, for example : how do I load and interact with an environment pickle file. How do I draw my own env, etc. Its a non exhaustive list, but we can get started with some, and user contributions will enrich it with time !

If you have some of such "How do I....?" tasks in mind, please include them in the comments.