Update documentation for Dask cuDF

[rjzamora]

Description

General documentation update for Dask cuDF:

• Adds README.md file to dask_cudf (this is currently a symlink to cudf's README, which isn't terribly helpful)
• Emphasizes direct usage of the dask.dataframe API (rather than the explicit dask_cudf API)
  • Including the to_backend API
• Advertises query-planning support
• Includes a simple Dask CUDA example (and best-practices link)

Checklist

• [ ] I am familiar with the Contributing Guidelines.
• [ ] New or existing tests cover these changes.
• [ ] The documentation is up to date with these changes.

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[rjzamora]

cc @quasiben @pentschev @wence- @charlesbluca @madsbk (happy to get feedback from any of you with time and interest)

[rjzamora]

This is a huge improvement over the README we currently have in dask_cudf, so I will mark this as ready to merge now. That said, I'm happy to follow up with further revisions if others find problems or have further suggestions.

[rjzamora]

/merge

[rjzamora]

Oops, just realized I never got a proper cudf-dask-codeowners approval. @pentschev - Does this seem good to go?

[bdice]

Oops. I didn't realize my approval would merge this PR. @rjzamora Can you apply my suggestions here? https://github.com/rapidsai/cudf/pull/16671#discussion_r1735013275

Also we need to make sure this information is all in sync with the dask-cudf docs, which don't mention query planning and other important topics. https://docs.rapids.ai/api/dask-cudf/stable/

[rjzamora]

Oops. I didn't realize my approval would merge this PR

Yeah, my fault! No worries - I will submit a follow-up today to update the API docs and I'll include your suggestion to use consistent language.