Feature/ff pm slides

@stefaniekiemle I know you already started working on the slides, but I couldn't find any branch on here. So I just added a bunch of stuff to some slides here.

You can use as much or as little of this as you please.

The images need to be resized and formated, and the latex equations aren't rendering properly yet. I also put a pdf of the old slides in the img folder (take it out before merge), but it can help as a reference. None of the equations in the old presentation are done with iguanatex unfortunately.

closes https://git.iws.uni-stuttgart.de/dumux-repositories/dumux-course-orga/-/issues/9