ISIS docs: PATH addition

[michaelaye]

It would help if

https://astrogeology.usgs.gov/docs/how-to-guides/environment-setup-and-maintenance/installing-isis-via-anaconda/#__tabbed_4_2

gets a small section on adding ISISROOT/bin to the path, otherwise all ISIS tools are not findable.

Also, the trick on how to add it to an existing conda env could be added.

I can submit a PR if you let me know which file I need to edit.