Add building man page on Windows

Add building man page on Windows

Use rst2html to build html from rst. Ignore errors,
this is not fatal and affects only MSI build.

Modify MSVC GitHub Actions to install python3/rst2html
and add html man page to artifacts.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Samuli Seppänen <samuli@openvpn.net>
Message-Id: <20210825164752.808-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22771.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit d814581a9375713a0a4d70f751f84e750a5d661a)