Improve Documentation

[lruthotto]

We should improve the documentation: http://juliainv.github.io/jInv.jl/ Some ideas are

• write a short overview about each module on the respective site
• tutorial about how to setup and run an inversion (choosing mesh, linear solvers, etc)
• tutorial about how to run inversions in parallel
• tutorial about how to code your own forward problem
• add a page about who is developing and using jInv as well as collecting papers about/using jInv
• webdesign (put logo?)

Any more ideas? All problems are up for grabs. Just let others know when you are getting one on or want it to be checked. The changes need to be made in docs/src (you can download everything or edit online). More info about how to use Documenter.jl is here: https://juliadocs.github.io/Documenter.jl/latest/