make documented pgc example

[PimLeerkes]

maybe jump'n'run

[ToxiCore]

The documentation tells users a little bit on how to use the pgc module, but not what pgc stands for or means. What is it used for? What is a "delta function"? It also explains that pgc has its own kind of State and Action, "(this function uses the pgc classes State and Action instead of the ones from stormvogel.model)", but not how and why they differ. These things should all be clarified.