Adds new package options:
* enableDocs
* enableHaddockProgram
to control whether to build Sphinx docs, and GHC haddocks and the
haddock program.
Unfortunately currently the building of the `haddock `program
and generating GHC docs are mixed into one option, see:
https://gitlab.haskell.org/ghc/ghc/-/issues/20077
Making Sphinx docs disableable, and disabling them by default
for Musl and cross builds, makes it much easier to provide these
builds without having to support Sphinx's enormous dependency
tree for those ways of building.
f4e62a996f