doc/workflow: prefer typesafe containers

doc/workflow: prefer typesafe containers

The typesafe containers have been around for quite a while now and
haven't gone up in a blaze of flames, so let's add a "strong
recommendation" to use them for new code and refactors.

For the nhrpd custom lists I'm already working to remove them; meanwhile
the old skiplists are primarily used in RFAPI (4 users outside of that),
so those could be next.

What remains are the old `list_*` and `hash_*`, which have >300 and >100
users respectively, making them a much harder problem to tackle.  And
the new hash implementation doesn't have the same level of
debug/introspection yet (it's on my TODO.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>