Hadrian: introduce ways to skip some documentation targets
The initial motivation for this is to have a chance to run the binary
distribution rules in our Windows CI without having to install
sphinx-build and xelatex there, while retaining the ability to
generate haddocks. I just ended up extending this idea a little bit so
as to have control over whether we build haddocks, (sphinx) HTML manuals,
(sphinx) PDF manuals and (sphinx) manpages.
(cherry picked from commit 8442103aa575dc1cd25cb3231e729c6365dc1b5c)