Adapted readme according to list feedback
Adapted readme according to list feedback

* unified links to be directory-style
* renamed all sites to libreoffice (except central infrastructure)
* removed obsolete info (gnome session manager etc)
* added hint for gfx perf on linux
* better mac system requirements
(cherry picked from commit c60a7a9fe9a6ac0897af92fcc812a38ce10da046)

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core