Add instructions about the ICECHUNK_ environment variables and how they work with CDK logic

Users would benefit from instructions on how to use the ICECHUNK_ environment variables depending on whether they want CDK to create the bucket or if the user wants to specify the name on an existing buket

No specific documentation file is named. Start by locating the project documentation and CDK configuration, then identify the ICECHUNK_ environment variables and the paths for CDK-created versus user-specified existing buckets. Done means documenting both workflows clearly and checking that the instructions match the current configuration.

Written by the indexing model from the issue text.