Add baseline images as a release asset for each release

[seisman]

Since we migrated all baseline images to DVC (#1036, #1131), the source and binary distributions of PyPI and conda-forge no longer contain baseline images (#1086).

These baseline images are all stored in the DAGsHub repository (https://dagshub.com/GenericMappingTools/pygmt), so we should be able to download these baseline images of any specific version at any time. However, it's still possible to delete snapshots of these baseline images using dvc's gc command. It would be better to add all baseline images as a release asset, so at least we have a backup of the snapshots.