Added Deep Reinforcement Learning example

[Guillem96]

Add a Deep Reinforcement Learning example to demonstrate the JAX versability across multiple domains.
The example implements a simplified version of the algorithm described in Playing Atari with Deep Reinforcement Learning.
For simplicity, the agent is trained in the CartPole gym environment.
To demonstrate that the example works properly I attach a plot reporting the timesteps that agent holds the pole along with the different episodes.

We can see as the number of training episodes increases the agent is capable to hold the pole for longer times.

For the example, I tried to apply the best JAX good practices. as I could

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[Guillem96]

@googlebot I signed it!

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