MAINTAINERS: Add Juan Quintela to developer guides review

Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20230419163457.17175-1-quintela@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>