This puts the generated files where they are for the make system.
This is in preparation for further meson-mdbook stuff.
Change-Id: I934df6854a80af5ccf381cf1da0bda0187a8bcfc
this notably does *not* install the `nix3-manpages` manpage the old
system generated, mostly because that page was empty and just a bug
with a coat of documentation paint.
Change-Id: I7a4248a72e7bb5e0cc925a6311a33b6b72589569