Adding tasks and default scoring functions

[felixchalumeau]

Let's add tasks to QDax (rastrigin, sphere, arm, etc...) and provide default scoring functions for them as well as for Brax environments.

[Lookatator]

TODOs left:

• [x] updating tests
• [x] remove type aliases such as PlayStepFnType
• [x] slight update of the documentation

[felixchalumeau]

Closed by #73