Add ppo.py documentation

[vwxyzjn]

This PR adds more PPO documentation and tracked experiments.

Checklist for ppo.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 cleanrl W&B team (required).
• I have tracked applicable experiments in cleanrl/benchmark with --capture-video flag toggled on (required).
• I have updated the documentation accordingly.
  • I have explained note-worthy implementation details.
  • 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.
  • I have added links to the tracked experiments.
• I have updated the tests accordingly (if applicable).

[gitpod-io]

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/vwxyzjn/cleanrl/GsqzdrsLbvPGBP9hU4F2j22DRVvt
✅ Preview: https://cleanrl-git-ppo-docs-vwxyzjn.vercel.app

[vwxyzjn]

Merging as is but happy to revisit.