Auto-generate reference documentation for cabal-install in reSt format

`cabal-install` has an auto-generated manpage these days (see #2877 -- thanks @mmakowski!). It would be nice if we could use the same amazing technology to also generate a reference page in reSt format to be included as part of our Sphinx documentation. This likely necessitates moving `Cabal/doc` to `cabal-install/doc`. Once this is implemented, next logical attack vectors are auto-generated reference pages for `~/.cabal/config` and the package description format.