doc, alternator: split "experimental" features from "unimplemented" ones
doc, alternator: split "experimental" features from "unimplemented" ones

Currently in docs/alternator/compatibility.md experimental features
and unimplemented features are bunched together under one heading
("unimplemented features"). In this patch we separate them into two
sections. This makes the "unimplemented features" section shorter,
and also allows us to link to the new "experimental features" section
separately.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

Closes #10893

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla