manual: contributing: move and expand autobuild section

This patch moves the section about autobuild fixing up, and expands it with
more details about what autobuilds are, and how you can help with it.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>