Remove pregenerated manpages from the repo

Remove pregenerated manpages from the repo

We don't need them in the repo, it's sufficient if we pregenerate them
while preparing the tarball.  That way we don't have overhead while
modifying them but they are still available for installations without
Sphinx.

I assume that this will make rebases and cherry-picks across branches
easier, with less trial and error churn required in the CI.

It's implemented in the way that we build the manpages only when we
either have pregenerated pages available at the configure time or
sphinx-build is installed and working.