GPU support coverage table

[GenevieveBuckley]

We should have a table in the documentation explaining what has GPU array support and what doesn't.

It could be similar to our table explaining function coverage in dask-image vs scipy.ndimage: http://image.dask.org/en/latest/coverage.html

It might make more sense for this table to list subpackages instead of functions (eg: imread, ndfilters, ndmorph, etc.). If we have a situation where some functions have GPU support and some don't in the same subpackage, then we might list that in detail.

[GenevieveBuckley]

Here's what's currently available https://github.com/dask/dask-image/issues/133#issuecomment-788515682

GPU support with cupy:

* imread
* ndfilters
* ndmorph
* ndinterp (nice work by Marvin)

No GPU support:

* ndfourier
* ndmeasure