Update FeI2 doc

[kbarros]

This removes all classical dynamics from our intro tutorial. Now uses minimize_energy! to find the ground state and LSWT for the S(q,omega) calculation.

To-do in a follow up:

• Perform 3-fold rotational averaging to illustrate beautiful agreement with experimental S(q,omega) data.
• Create new tutorials that illustrate the power of classical dynamics (e.g., thermal fluctuations in a spin liquid phase, a quench into CP^2 skyrmions, driven-dissipative dynamics, ...)