[docs] replace dask-lightgbm reference with LightGBM

[jameslamb]

Following the discussion in dask/community#104, the maintainers of dask-lightgbm agreed to upstream that project into LightGBM: microsoft/LightGBM#3515.

This week, the first version of LightGBM that includes a Dask interface was released.

• https://pypi.org/project/lightgbm/
• https://github.com/microsoft/LightGBM/releases/tag/v3.2.0

Now that the LightGBM Dask integration has been released, this PR proposes removing references to dask-lightgbm and replacing them with references to LightGBM.

Other Related Work

I've also opened dask/dask-lightgbm#30, proposing that dask-lightgbm do one more release that raises a warning directing users to LightGBM.

[jrbourbeau]

Thanks @jameslamb!

[TomAugspurger]

Thanks James (and James)!