doc: comment out most of "how to install" page, which turned out to be a troll

comment out System-wide Installation, Virtualenv-based Installation sections.

replaced "pants" with "pantsbuild.pants" in the commented-out text.

Bring back the "Pants is not an install-able" blurb we had before. Slightly updated; the old text assumed Pants lived in twitter/commons.

Bugs closed: 114, 158

Reviewed at https://rbcommons.com/s/twitter/r/414/