Add default index.html to each Kafka Tutorial

Kafka Tutorials have a URL of the form `https://kafka-tutorials.confluent.io//<tool>.html` e.g. `https://kafka-tutorials.confluent.io/rekey-a-stream/ksql.html`.

It would be nicer UX if a user strips the `<tool>.html` e.g. to `https://kafka-tutorials.confluent.io/rekey-a-stream/` they were taken to the default version of the tutorial (or at worst, the KT index page). At the moment they're dumped to an S3 error.