Document which functions need to be implemented for new concrete types

[jschepers]

For example, for implementing a new concrete design one also needs to implement a size and generate_events function. This is documented in a HowTo in the docs (https://unfoldtoolbox.github.io/UnfoldSim.jl/stable/generated/HowTo/newDesign/) but it would be great if this could also be found in the code e.g. in the implementation of the corresponding abstract type.