CLI docs for `avn service sstableloader` command

[ftisiot]

What should we add to our content collections?

Documenting the avn service sstableloader that Aiven provides for Cassandra services

Please fork the cli-avn-service branch, have a look at the integration page for an example

Is this something you would work on yourself?

[ ] I plan to open a pull request for this

[encima]

Command purposes are described here: https://help.aiven.io/en/articles/3147884-using-sstableloader-to-migrate-your-existing-cassandra-cluster-to-aiven (for reference)