Reorganize forcefield tests

[esoteric-ephemera]

In the hopes of progress towards closing #1376

• Create separate forcefield dependency groups based on compatible install environments
• Skip forcefield tests when not installed (probably best practice anyways? we need to be more careful about this when developing)
• Separate forcefields by test group in CI
• MatterSim tests

Docs:

• Expanding user forcefield docs with FAIRChem install instructions + more metadata
• Added user add-on info
• Added dev forcefield maintenance strategy guide

Tangential:

• Bump numpy<3 (except in a few edge cases)
• Fix APIs to Nequip, Allegro, and FAIRChem

[JaGeo]

@esoteric-ephemera thanks again for this work! Could you maybe add a few comments directly to our pyproject yaml or elsewhere to make sure that we do keep this consistent in the future?