Docs: Cassandra data source documentation

[scsmithr]

Description

External table:

CREATE EXTERNAL TABLE basic
	FROM cassandra
	OPTIONS (
		host = 'my.host',
		keyspace = 'test',
		table = 'bikeshare_stations'
	);

External db:

CREATE EXTERNAL DATABASE db
	FROM CASSANDRA
	OPTIONS (
		HOST = 'my.host',
	);

And read_cassandra.