Add dqn_atari.py documentation

[vwxyzjn]

Checklist for dqn_atari.py:

• I've read the CONTRIBUTION guide (required).
• I have ensured pre-commit run --all-files passes (required).
• I have contacted @vwxyzjn to obtain access to the openrlbenchmark W&B team (required).
• I have tracked applicable experiments in openrlbenchmark/cleanrl with --capture-video flag toggled on (required).
• I have updated the documentation and previewed the changes via mkdocs serve.
  • I have explained note-worthy implementation details.
  • I have explained the logged metrics.
  • I have added links to the original paper and related papers (if applicable).
  • I have added links to the PR related to the algorithm.
  • I have created a table comparing my results against those from reputable sources (i.e., the original paper or other reference implementation).
  • I have added the learning curves (in PNG format with width=500 and height=300).
  • I have added links to the tracked experiments.
• I have updated the tests accordingly (if applicable).

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