docs: Support for generating man(8) pages

docs: Support for generating man(8) pages

The manpage rules are very repetative, because of the section number being
present in the filenames.

Instead of adding another set of 3 rules, switch to using a metarule to
automate the repetative action.  New rules for different manpage sections can
be added simply by extending the $(foreach)

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>