add missing imports for dask-cuda docs

[jameslamb]

Contributes to #374 .

Fixes the following in dask-cuda docs at https://docs.rapids.ai/deployment/nightly/tools/dask-cuda/:

• marks Python notebook cells as Python code, so that:
  • they'll be Python syntax-highlighted when rendered
  • linters can cover them in CI
• adds missing imports
  • it isn't obvious from this page that LocalCUDACluster is something you import from dask_cuda
• removes if __name__ == "__main__" stuff in example
  • so that the example code is runnable in a notebook or REPL without the extra friction of writing it to a script