Update data_management.md

[Haitham-ghaida]

Update How do I change my data directory in a conda environment - for Unix

The method in the current docs doesn't work or is outdated, the files need to be in etc/conda/, not at the root of the env directory. I also added what the files should look like to make it clearer and added the commands to ensure script execution permissions.

[michaelweinold]

Thank you, @Haitham-ghaida for your contribution! I have now merged it into the main branch 🎉