modify conda guide

[ChristinaLK]

• remove "install on the fly" instructions?
• add notes about modifying environments, per this suggestion from Pat:

Some thoughts I wanted to express, is that while the documentation is clear for creating a packaged python environment from scratch it does not have a recommended route to add/remove packages from a conda/python environment once created. May be a good note for the documentation to add a recommended method of adding or removing packages from our environment.