Docs: add GPU support info to coverage table

[GenevieveBuckley]

Closes #197

This PR adds information about whether functions have GPU support (i.e. can use dask arrays backed with cupy chunks as input to the function)

• File: https://github.com/dask/dask-image/blob/main/docs/coverage.rst
• Webpage: https://image.dask.org/en/latest/coverage.html

[GenevieveBuckley]

This PR is unrelated to any GPU/cupy code, so I'm going to ignore the GPU CI build error. I've opened #302 so we can fix the GPU CI problem separately.

[jakirkham]

Thanks Genevieve! 🙏