Add a section on a more complex example in the "how to implement a sampler" tutorial

[torfjelde]

In #468 we've introduced a tutorial on implementing samplers, but atm the inference alg is very simple. We should add a section at the end of the tutorial demonstrating how to "extend" this simple MALA sampler to the autoMALA sampler (see https://github.com/torfjelde/AutomaticMALA.jl)