doc: process for adding a maintainer if the candidate does not have a role

There is no clear way for a candidate to assume the role of Maintainer
if the candidate is neither a Contributor nor a Collaborator. To avoid
disputes and confusion, candidates who are neither Contributors nor
Collaborators must be approved by the TSC before they can assume the
role of Maintainer.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>