Add unmodeled dynamics section to feedforward

[mark-rifkin]

On the feedforward document page, it would be helpful to add a section about feedforward for unmodeled dynamics. The paragraph below somewhat implies that such a section should be there, but it seems not to.

There are two types of feedforwards: model-based feedforward and feedforward for unmodeled dynamics. The first solves a mathematical model of the system for the inputs required to meet desired velocities and accelerations. The second compensates for unmodeled forces or behaviors directly so the feedback controller doesn’t have to. Both types can facilitate simpler feedback controllers. We’ll cover several examples below.

The one in Controls Engineering in FRC is helpful but maybe a bit complicated for the purposes of the docs. In addition to the information included in the book, I would also find it helpful to have something on the use cases unmodeled feedforward (specifically as it relates to steady state error).