Documentation: Add info on GPU CI to contributing guide

[GenevieveBuckley]

We might like to update the dask-image contributing guide to include:

1. Information about the GPU CI bot (see here, and/or
2. A link to the main dask contributing guide.

This would require editing the dask-image CONTRIBUTING.rst file to add new information.

[GenevieveBuckley]

This was suggested by Charles here: https://github.com/dask/dask/issues/7975#issuecomment-1192121085