This reverts commit 6bfb5a20.

The sandbox feature is already documented (see #1430).
Additionally, the documentation added in this commit is misleading:
'runhaskell Setup.hs sandbox CMD' doesn't work.

See jgm/pandoc#976.

The docs/website sorely need to be reworked, but this will do for now.

Makes things a little simpler in GHC's build system if libraries are in
the same directory as their name.

At the top level we now have cabal/ and cabal-install/

The chapter on installing packages still needs to be rewritten
to describe the cabal tool, rather than runhaskell Setup.hs