[tune][docs] remove outdated ScalingConfig references

[matthewdeng]

Why are these changes needed?

1. Remove ScalingConfig from Tune documentation.
2. Remove Ray cluster (client?) section from the user guide.
3. Link Ray Train Hyperparameter Optimization guide.

Related issue number

#44914

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 :(

[justinvyu]

Thanks for the cleanup!