Point to the MNE installation instructions for getting MNE to run

[hoechenberger]

We've put a lot of work into creating the MNE installation instructions because there are sooo many potential issues users can run into. Instead of simply suggesting users run conda install mne or pip install mne here, I'd just say: see the official MNE installation instructions and then come back here again.

Edit: And once we have a conda-forge package of MNE-ICALabel, we can include it in the installer and things will become even easier for users!

[mscheltienne]

I think we can wait for the conda package and inc. in the installers before refactoring the installation page in the documentation.