Expand ParticleNet/Weaver Documentation

[aperloff]

In light of arXiv:2202.03772, I have several questions:

1. Will the ParT network be integrated into CMSSW? Is this already underway?
2. Should the "ParticleNet" documentation be renamed/moved to be more about retraining a class of networks using Weaver?
3. Can information about ParT be added to the "ParticleNet" documentation?
4. Should we add a new document about attention mechanisms and then cross reference that with the possible ParT documentation so that the information is split up into bite-sized chunks?