update installation instructions

[orbeckst]

The installation instructions https://www.mdanalysis.org/hole2-mdakit/getting_started.html#build-and-installation and README need to be updated to include the pip installation from PyPi.

Given how important hole2 is, we should explicitly indicate how to install it:

• Linux/macOS: can use conda-forge (conda -c conda-forge install hole2) or source install from https://github.com/osmart/hole2
• Windows users need to install their own version of hole2.

Eventually we should have a conda-forge package for this MDAKit.

Old issue text:

~The docs https://www.mdanalysis.org/hole2-mdakit/getting_started.html#build-and-installation state that there's no conda package available. However, the README shows the recently added conda-forge package installation.~

The sphinx docs should be updated to show ~the conda installation first (because of the ease to pull in hole itself),~ then pip, then source install.~

[orbeckst]

We should consider making the hole2 conda-forge package a dependency for mdahole2 (at least for Linux and OSX) so that one gets all needed packages in one go.

Will open issue.