Update the genotypes and subtypes modding page

The current genotypes and subtypes modding page (https://wiki.cavesofqud.com/wiki/Modding:Genotypes_and_Subtypes) is pretty sparse, and its primary advice is to review genotype and subtype mods in the Steam workshop (which are mostly heavily out of date). It would be better for this page to actually include a small end-to-end example of adding your own genotype or subtype to the game.