[Buildroot] [PATCH 0 of 7] manual: rework Contributing chapter

[Thomas De Schampheleire]

This patch series reworks the "Contributing to Buildroot" chapter in the
manual. According to the latest Buildroot Developer Days, we want to promote
contributions that are not feature patches, like fixing autobuild failures,
closing bugs, working on items from the TODO list, etc. This series reorders
and expands the chapter with that desire in mind.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire at gmail.com>

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v2:
- integrate rewording comments by Yann, Samuel and Arnout (thanks!)
- add new patch modifying the commit message template