modify and add index.md for each sectionin user docs for better under…

[Svanvith]

…standing and look and feel. Check links and fix wrong

[micbar]

Exhaustive list of all broken links found:
  - Broken link on source page path = /docs/next/user/desktop-client/:
     -> linking to ./windows/ (resolved as: /docs/next/user/desktop-client/windows/)
     -> linking to ./macos/ (resolved as: /docs/next/user/desktop-client/macos/)
     -> linking to ./linux/ (resolved as: /docs/next/user/desktop-client/linux/)
     -> linking to ./common-functionality/ (resolved as: /docs/next/user/desktop-client/common-functionality/)

[Svanvith]

Links are fixed @micbar