doc: Use generic rules for man/html generation

doc: Use generic rules for man/html generation

Prior to this patch, the Makefile.am needs to be modified multiple
places to add a new man or HTML page to be generated.  Since it is not
too often we modify this, it is easy to miss these finer details.

This changes the man and HTML generator rules to be more generic and use
variables as many places as possible.  Also moved all the lines which
should not need to be changed as much towards the bottom-half of the
file.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210630185134.144826-1-openvpn@sf.lists.topphemmelig.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22604.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>