Add TorchSpatial to list of related libraries

### Issue

We should consider adding [TorchSpatial](https://github.com/seai-lab/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.