doc: improve index page and navigation

[petrasovaa]

• Improved index page
• jupyter_get_started.md removed, content moved to interfaces_overview.md
• remove interfaces from Getting started, we could put interfaces_overview.md back there, but it would be linked from the main navigation again, and that messes up the label of the page in the navigation.
• remove wxguiintro.md, replaced in the navigation with helptext.md, which fits better as a practical guide how to start, the intro was anyway lame.
• link from interfaces_overview.md to individual pages done with buttons to highlight them better.
  

TODO: need better GUI screenshot with single layout.