Clarify instructions for tutorials that instruct to use docker-compose in background mode

For example: https://kafka-tutorials.confluent.io/creating-first-apache-kafka-producer-application/kafka.html

In step 2 we instruction to launch CP w/ `docker-compose up -d`

In step 9 we mention shutting down by `Now close the consumer with a CTRL+C then the broker shell with a CTRL+D` but we've instructed to run docker in the background, so this instruction will be confusing.

For any tutorial where these instructions exist, we should modify shutdown instructions for docker to something like:

> Shutdown the Confluent Platform components with `docker-compose down -v`