manual/getting started: rename and move 'Getting involved' section

The Getting Involved section is actually an overview of the ways to interact
with the Buildroot community, which is useful for developers _and_ users.
Therefore, this patch moves the section from the Developer Guide to the
Getting Started section, and renames it to 'Community Resources'.

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