doc: contribute: project roles: modify description of release team

We modify the content around the Release Team formation,
and maintenance, describing in detail how maintainers
may be added to the team. Add a comment about Release
Managers that stresses that they must be members of the
team. Add details on how the TSC shall ensure a functional
release team. Add details on how the release team will
be picking release managers (cadence and selection process).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>