[DOC] Update ML docs to reflect recent changes

[sarahyurick]

After the changes in #962, we should update some of our machine learning docs such as https://dask-sql.readthedocs.io/en/latest/machine_learning.html to include backend agnostic ML models. Furthermore, it would be a good idea to revisit other ML changes made recently (#832, #855, #886, #926, #1007, and #976) and consider updating the documentation accordingly.

We still have some debugging to do on the GPU-ML side (see #943 and #1020), so it might be a good idea to make a note of known limitations and/or include working examples once these issues are addressed.

I also think this would be a good chance to close some other existing issues around our ML docs, like #130 and #397.

[sarahyurick]

On the GPU XGBoost side, it would also be worth emphasizing the gpu_hist parameter.