Bring handbook up to date

There are a few larger TODOs in the handbook:

• Check and update chapter 2 Getting started (@hommel): maybe split in two chapters ("Getting started" including the quick guide and something else...) or rename chapter, add grids (@kweis), delete mentioning of dumux-devel
• Check and update chapter 3 Tutorial (@nedc): explain check out of dumux-course with the dumux-course tag, mention that dumux-course will be updated soon to work with 3.0 release and that examples will be added in 3.1 release, delete tutorial
• Update chapter 4.1, 4.2, 4.3 plus change file for naming conventions to md-file (@kweis)
• Update chapter 5.4 Restart (@timok)
• Add Input/Output chapter with paraview, gmsh... Move 5.5 Grid Handling to this chapter (@leopold.stadler, @utz)
• Write subsection on grid (e.g., cornerpoint, @martins) in 5_grids.tex
• Write subsubsection on temporal discretization in 5_models.tex (@felixw, with help from @holle)
• Write subsubsection on MPFA (@martins) in 5_spatialdiscretizations.tex
• Revise subsubsection on Staggered grid (@melaniel) in 5_spatialdiscretizations.tex
• Revise section about implementation structure and revise figure in 5_stepsofasimulation.tex (link to main file from course) (@melaniel)
• Check and update 5.4 Property System (@seitz)
• Check and update 5.5 Grid Handling (@nedc): ICEM raus
• Write section on how to use DuMuX in parallel (@leopold.stadler, @utz): mpirun, only AMG, Multidomain and freeflow not in parallel, depends on grid (dune)
• Add section on gas mixing laws
• [ ] Add chapter on physics of porous media flow (e.g. pc-sw curves) (@holle)
• Link to wiki about changes from 2.12 to 3.0 (@becker)