Webpage reorganization to make beginners info easier to find / undestand

Inspired by #93 (closed) we started restructuring the documentation...

Here is a list of things we think must still be done...

• collect different installation notes in a unified section
  • add windows WSL binary installation (@robert.kloefkorn)
  • add windows src installation
    • WSL + Linux source
    • Native Windoze
  • add MacOS src installation (@samuel.burbulla)
• getting started section (as new first level entry)
  • links to installation (see above)
  • first steps C++
  • your first module
  • link to book
• update pythonbinding.md
• restructure / organize tutorials
• merge grid module overview & grid feature lists
  • reorder grid modules (requires module sorting via name & weight)
• collect further documentation (cheatsheet, ...)

fixes #88 (closed), #93 (closed), #87 (closed), #67 (closed), #64 (closed), #66 (closed), #46 (closed), #42 (closed), #23 (closed)