Docs: Restructure the Embedded Linux article to improve readability

* Split out the Inputs chapter into its own page
* Cleanup the Inputs page to improve readability
* Remove outdated bits of info from the Embedded Linux article itself

Task-number: QTBUG-75725
Change-Id: Ia1031da92c302d78903d22199f7f13cde3a512bc
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>