[[forum/refresh_and_setup]] indicates some confusion between --setup
and -setup. Both work, but it's clearer if we stick to one in
documentation and code.
A 2012 commit to [[plugins/theme]] claims that "-setup" is required
and "--setup" won't work, but I cannot find any evidence in ikiwiki's
source code that this has ever been the case.
It will set up an ikiwiki instance tuned for use in blogging.
As part of this change, move the example sites into /usr/share/ikiwiki so
they are available even if docs are not installed.