Change conda install instructions to conda-forge channel #1104
The Astropy conda packagers recently moved Gammapy and all the other conda packages previously maintained by the Astropy team to conda-forge.
In the past years, conda-forge has emerged as the most popular / well-organised solution for conda packaging / build / distribution. To quote from https://conda-forge.org/
Here's the conda-forge package for Gammapy:
I think this is the long-term conda solution for Gammapy as an open source project for the coming years (irrespective if others like CTA start building or distributing conda packages).
This PR updates the Gammapy docs to use