Add ScyllaDB support to documentation

[andrii-kysylevskyi]

Description
ScyllaDB is a drop-in, more performant replacement for Apache Cassandra. It works out of the box with django-cassandra-engine simply by replacing the Cassandra connection string with a ScyllaDB one. Also, this project has already replaced cassandra-driver with a scylla-driver in commit 6af682a (Release version 1.7.0), which only helps ScyllaDB's support. Also, Scylla's image seems to be used in tox.yml

Suggestion
Mention this compatibility in the following places:

1. README file
2. docs/guide/getting_started.md
3. docs/indexmd

Additional context / Benefits:

For new django-cassandra-engine users, this makes it clear that they can use ScyllaDB out of the box if their organisation already runs it, without needing to rely on Cassandra directly.
For existing users, it highlights that they have the option to migrate from Cassandra to ScyllaDB if performance or scalability becomes a concern, knowing that their Django app will continue to work seamlessly.

Testing notes:
I have run the tests against ScyllaDB, and they all succeeded

I am happy to work on these changes