Better document / advertise the dispatching mechanism

[m-albert]

I think that for most of dask-image's functionality the documentation 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 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?

[jakirkham]

Sounds like a good idea to me!