docs: let sphinx build man pages for better integration and less cruft

docs: let sphinx build man pages for better integration and less cruft

rst2man is only good for use in projects that don't use sphinx
already, as there it can help to avoid bringing in the full sphinx
dependencies and be easier to manage (as long as it stay small).

But we already use sphinx, so there's no point in managing the manual
pages in a separate, semi-related way that is quite restricted as we
have no access to sphinx infrastructure like conf.py defined
variables and helpers for things like the current version.

Besides that, we're rather big, so the complexity of sphinx can
shine, e.g., see the diffstat report from switching out rst2man for
sphinx:

22 files changed, 74 insertions(+), 347 deletions(-)

so 273 lines less in total is rather nice.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>