doc: Clarify dim cherry-pick usage

The section about `dim cherry-pick` precedes the docs split in a committer
and maintainer sections. Having it in the committer/getting-started.rst is
misleading, because only maintainers are supposed to cherry-pick commits.

Suggested-by: Simona Vetter <simona.vetter@ffwll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>