DOCS: Fix dqn agent example

[tomzhengy]

Related to JOSS review openjournals/joss-reviews#10470

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1. The README policy-representation examples used agent registry keys that do not match the implemented make_agent registry:

   make_agent("boolean/mlp", ...)
   make_agent("dt/mlp", ...)
   make_agent("dqn", ...)

   I updated those examples to the registered keys mlp/bool, mlp/dt, and dqn/mlp.

2. The DQN example also used layers, but DQNAgent reads q_layers. I updated the config key so the example matches the current public API.

I checked this locally by constructing mlp/bool, mlp/dt, and dqn/mlp agents, then running agent.init(...) and agent.policy(...) on a CartPole observation for each.

Please edit this PR and/or clarify if needed!