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

[23Skidoo]

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.

[andreasabel]

Maybe a GSoC project?