Docs: Cassandra data source documentation

## 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`.