Documentation & packagedown-page

[tlorusso]

The documentation of the statR should be improved, the actual documentation consists in a single vignette:

https://statistikzh.github.io/statR/

ToDos (dev-branch):

• [x] Split up the intro-vignette into one for graphics and one for excel-generation
• [x] Add examples of how to create color-blind friendly charts
• [x] Vignette for the my_sum fuction / color functions
• [x] rebuild entire pagedown site
• [x] Design of the packagedown page @tlorusso -> Font / remove orange accent color