docs: Cleanup old documentation

* Use active voice to improve readability
* Restructure the topic to reduce the number of sections
* Simplify parts of the content to make it easier to skim

Change-Id: Ic8b327acfc3c3d6435324ba219bd5eebcd9d159a
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>