spack doc should be on docs.rc, not here

[maggiemcfee]

Our primary Spack doc should be on docs.rc, not here. This is all doc, not code https://github.com/fasrc/User_Codes/blob/master/Documents/Software/Spack.md

Docs on docs.rc should link out to github code as appropriate, but putting documentation here fragments our docs and makes searching by users difficult. Every 'thing' should have a page on docs.rc, even if it's primarily to link to code on github. Spack only had a link out from an overview page. (example: searching for "spack install" only leads you to the Rocky8 Transition page which has a link here, but no in situ information which does exist, but on another website)

Nutshell: 100% of https://github.com/fasrc/User_Codes/blob/master/Documents/Software/Spack.md is documentation and should be on https://docs.rc.fas.harvard.edu/kb/spack/