Add more Example Notebooks & Enhance Documentation

[Ceyron]

• Explaining Substepping (how to make nonlinear steppers stable at high $\Delta t$
• Explaining Difficulty Steppers
• On conventions in Exponax (state array layout, fft is always rfft, batch then time (vmap over rollout), only real-valued problems, same number of dof in each direction(hence axis of rfft halving does not matter), same domain_extent in each dimension)