Notation for KFE should not use `v`

[jlperla]

We should use a different function symbol for all of our examples with the KFE.

• [x] Use $\pi$ for payoff function throughout the package docs and appendix, instead of $f$. Also, use $f$ for distribution from now (Resolved by 787f3b0)
• [x] Change anything with v to maybe g f in these, throughout. This includes the notebook and the docs (e.g. https://quantecon.github.io/SimpleDifferentialOperators.jl/dev/#Finding-stationary-distribution-from-the-Kolmogorov-forward-equation-(KFE)-1) (Resolved by 455c24d)
• [x] There is a layout issue in the docs there as well, (Resolved by 455c24d)
• [x] And this uses f(x) as well, so make sure it is consistent (Resolved by 455c24d)
• [x] Also, note that there is a mu(x) here but we are in a constnat coefficient world (Resolved by 455c24d)