manual/getting started: rework 'Getting Buildroot' section

This patch reworks the 'Getting Buildroot' section of the manual as follows:
- be more specific on the Buildroot release cycle
- refer to the download page of the website for details on snapshots and
  git, instead of duplicating the information.

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