Add docs for Kafka SASL / SCRAM configuration to Apache Kafka Ingestion

If Kafka is configured with SASL / SCRAM, then Druid, which is the Kafka consumer, should pass the SASL / SCRAM credentials in the consumerProperties block of ioConfig section of the Kafka supervisor spec:
```
"ioConfig": {
"topic": "some_topic",
"consumerProperties": {
"bootstrap.servers": "host1:9092,host2:9092,host3:9092",
"sasl.mechanism": "SCRAM-SHA-256",
"security.protocol": "SASL_SSL",
"sasl.jaas.config": "org.apache.kafka.common.security.scram.ScramLoginModule required username=\"some_userID\" password=\"some_password\";"
}
"taskCount": 1,
"replicas": 1,
"taskDuration": "PT1H"
}
```

cc: @druid-matt , @petermarshallio

Might be a good issue for a community contributor.

Locate the Apache Kafka Ingestion documentation and find the configuration section for Kafka supervisor specs. Use the supplied consumerProperties example as the starting point, documenting the SASL/SCRAM mechanism, security protocol, and JAAS configuration, then verify the example matches the supervisor spec.

Written by the indexing model from the issue text.