Update documentation to remove `defaults` channel and suggest miniforge

[jashapiro]

Closes #712

Here I updated the documentation to give instructions on how to install conda via miniforge, rather than with the graphical installer on Mac & command line elsewhere. The instructions should be the same for all installs, so I consolidated the instructions into a single block, with only a note about Windows/WSL. Does that seem reasonable?

I also removed the GitKraken terminal references, as we have seen errors with that (maybe miniforge is better, but I doubt it!).

[jashapiro]

I recall briefly discussing at one point that we might have instructions "what if i already installed miniconda?" and I now think maybe it's fine to just deal with that on a case-by-case basis instead? Or, we could add a by default collapsed admonition with code, for example, to removing defaults from the channels list and other small tasks

I think we can do this on a case-by-case basis, yes. If they use our templates defaults won't be in the channel list, so that is a start, but we should be keeping an eye out for it in conda environment files and conda-lock files. But in general it should be pretty easy to deal with, and I would rather keep the docs simpler until proven otherwise.