Fixes https://github.com/scylladb/scylladb/issues/14598
This commit adds the description of minimum_keyspace_rf
to the CREATE KEYSPACE section of the docs.
(When we have the reference section for all ScyllaDB options,
an appropriate link should be added.)
This commit must be backported to branch-5.3, because
the feature is already on that branch.
Closes#14686