In the 5.1 -> 5.2 upgrade doc, include additional steps for enabling
Raft using the `consistent_cluster_management` flag. Note that we don't
have this flag yet but it's planned to replace the experimental flag in
5.2.
In the "Raft in ScyllaDB" document, add sections about:
- enabling Raft in existing clusters in Scylla 5.2,
- verifying that the internal Raft upgrade procedure finishes
successfully,
- recovering from a stuck Raft upgrade procedure or from a majority loss
situation.
Fix some problems in the documentation, e.g. it is not possible to
enable Raft in an existing cluster in 5.0, but the documentation claimed
that it is.
Follow-up items:
- if we decide for a different name for `consistent_cluster_management`,
use that name in the docs instead
- update the warnings in Scylla to link to the Raft doc
- mention Enterprise versions once we know the numbers
- update the appropriate upgrade docs for Enterprise versions
once they exist
