[FEA] Document CUDF host compression option

[revans2]

Is your feature request related to a problem? Please describe.
CUDF recently added in the option to compress gzip orc and parquet files on the CPU instead of the GPU using a thread pool.

rapidsai/cudf#17656

This is controlled by two environment variables.

• LIBCUDF_HOST_COMPRESSION_NUM_THREADS to set the number of threads that the pool will use (this is a static pool that appears to be shared by everyone).
• LIBCUDF_USE_HOST_COMPRESSION which controls is we should try and use host compression if we have a choice.

Right now only gzip is available, but that might change in the future. At a minimum we should have this as an option to use to debug if nvcomp is having issues or not.