Avoid reference to upstream Docker set-up

For Db2, the docs say this:

This procedure assumes that you are logged in as the db2instl user, which is the default instance and user name when using the Db2 docker container image.
Prerequisites

On the machine on which Db2 is running, the content in debezium-connector-db2/src/test/docker/db2-cdc-docker is available in the $HOME/asncdctools/src directory.

This needs adjustment for downstream docs:

• First section should rather discuss required permissions (which ones?)
• The second section should rather refer to the files in the distribution