Better document / advertise the dispatching mechanism

I think that for most of dask-image's functionality the [documentation](https://image.dask.org) is very informative. Especially because most functions can be used in the same way as their ndimage counterparts and for knowing which functions are available the [function coverage table](https://image.dask.org) does a great job.

However I feel that dask-image's super cool dispatching functionality is under documented / advertised.

@GenevieveBuckley @jakirkham What do you think about adding a page "Array backends" or "GPU support" (better names welcome?) to the documentation?