Improve docs by describing format of payoff matrices

[pevnak]

The current version of the documentation does not state, how the payoff matrices are structured. It took me quite while to found in notebooks this note
Note that we adopt the convention that the 1-st argument of the payoff function ui is player i's own action and the j-th argument, j=2,…,N, is player (i+j−1)'s action (modulo N).
While I understand that this might be known for GameTheory practicioners, I was lost. Writing this explicitly in docs with a small example would be beneficial.