[RLlib; docs] RLlib documentation do-over (new API stack): Main index page.

[sven1977]

Update, refactor, fix the main RLlib index.html page (for the new API stack).

• Fully geared towards new API stack.
• Simplified (only mention a few high-value features).
• Better overview tables within the tabs for algos, environments, and features.
• Redo RLlib overview diagram at bottom of page (also simplified).

Why are these changes needed?

Related issue number

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  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[simonsays1980]

LGTM.

[simonsays1980]

We should bring in here loading the pipelines from checkpoint, too and using them.

[sven1977]

We have another example, where we do that (the LSTM one, which requires the connector pipeline for a more sophisticated inference loop w/ state in/outs).