Update RLlib in 60 seconds example

[worldveil]

A few things we want to do here:

• Show example that is more customizable, and shows something resembling user code
• Point to Tune checkpointing docs
• Point to policy inference example

On the first point, this can be any existing example in codebase/github, no need to design a new one unless we need to.

Can also replace this with even better one in full revamp, we just need something incrementally better now.

[sven1977]

Page has been updated. Closing this issue now. Please feel free to post a PR with fixes for the ray/doc/source/rllib/index.rst file.