Add TorchSpatial to list of related libraries

[adamjstewart]

Issue

We should consider adding TorchSpatial to our list of related libraries: https://torchgeo.readthedocs.io/en/latest/user/alternatives.html

Fix

@neminwu @QianC95 would you be interested in opening a PR? I think this would be a good advertising opportunity for both of us.