manual/getting started: rework 'System requirements' section

This patch reworks the requirements section of the manual as follows:
- some general rewording
- move configuration editor dependencies above the download tools, as this
  is the first thing people come in contact with.
- move sentence regarding -dev packages to configuration editor dependencies
  and restrict to 'libraries'.
- clarify the download tools part.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>