Extend Kalman filter documentation

[cbrxyz]

Currently, there is some documentation on the odom_estimator (the unscented Kalman filter), but it is very wordy and math-heavy, which there is no need for. This only makes it harder to understand the system at a basic level and prevents quick changes to the system.

The documentation should have its complexity kept, but simpler language should be added to better explain concepts used by the Kalman filter system.