Show how Flatté can be derived from a 2-channel K-matrix

[redeboer]

Currently, the K-matrix tutorial only shows how a Breit-Wigner can be derived from a K-matrix for one channel with one pole and how it compares to a K-matrix with two poles. The tutorial can be easily extended to show how a Flatté lineshape derives from a 2-channel $K$-matrix.

[redeboer]

Currently being worked out in https://github.com/ComPWA/K-matrix-research/pull/19. Once done, we can port it to the AmpForm docs.