Rework example gallery

[Teschl]

This reworks the example gallery in the documentation. It's possible to add custom thumbnails to each tile now. I have reduced the notebooks down to four files. Once we decide how we want to design the thumbnails, we can close 36. I also added a reference to the tests in our README and resolved #46 (__init__ twice in docs).

I spend way too long trying to find this solution. Since nbsphinx is not the only extension that can generate galleries, I tried to implement a few of them before I finally figured out this, admittedly, very easy solution that is implemented now.

Seaborn for example uses Sphinx Gallery, and they were able to easily add custom thumbnails. I first tried to convert our docs to support Sphinx Gallery. This requires .py files that will in turn be converted to .ipynb files when building. That would mean either having the examples stored twice or not having any notebooks to work with. So I scrapped this approach again.

closes #18

I think we can also close 27 since that is all implemented now. Therefor that also resolves 30

[wkearn]

Looks great, @Teschl!

I edited your PR description so it wouldn't close all of the issues immediately.