docs: manpages: Strip table of contents from manpages

docs: manpages: Strip table of contents from manpages

After meson conversion the man pages started to contain the table of
contents.

In autoconf we prevented this by a 'grep -v ::contents' in the command
building the manpages.

A more cultured solution is to strip out the 'contents' docutils element
directly.

Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>