Improvements to PBT example

[crypdick]

Why are these changes needed?

Various improvements to PBT tutorial:

- rm references to Binder and Collab notebooks
- add missing pip reqs
- hide verbose cell outputs
- expanded and clarified table of math symbols
- updated colors of the plot to match the screenshot of the PBT figure
- fleshed out the narrative a bit more

Related issue number

N/A

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • 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.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(