Nexus: Add description of installing specific package version

[prckent]

Once a release has been made including #5742 , expand the documentation to include a description of how to install a specific tagged version (from Git) or from a downloaded QMCPACK release tarball.

[zeel2104]

I can work on this issue. Can you assign it to me?

[prckent]

Many thanks for your interest! You are very welcome to do this, but we need to get the v4.3.0 out release first. Hopefully this will only be a couple of weeks while the remainder of the Python packaging update goes through.