Change to markdown file, update information for Qt6 and the preferable
way of initializing the build environment. Remove license header to make
the file more readable.

Pick-to: 6.1 6.0
Change-Id: I730f5eba376ce5337979bcd86287f1cb174e9ac2
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>
Reviewed-by: Pasi Petäjäjärvi <pasi.petajajarvi@qt.io>