Add Myria3D to list of related libraries

[adamjstewart]

Issue

Not clear if Myria3D is still actively being developed, but if so, we should add it to our list of related libraries: https://torchgeo.readthedocs.io/en/latest/user/alternatives.html

Fix

@CharlesGaydon would you like to submit a PR to add this? We usually have a 1-year threshold before we decide a project has been abandoned, so if there are no plans to revive it, we don't have to add it.