Properly incorporate FCS into the build and publish pipeline

[cartermp]

This issue is to track the tooling RFC detailed here: https://github.com/fsharp/fslang-design/blob/master/tooling/FST-1008-publish-fcs-from-upstream-repo.md

[cartermp]

Done, we just need to update docs generation and get release notes over into the file here now.