doc/Makefile: rebuild rst docs if input files change

doc/Makefile: rebuild rst docs if input files change

For now the dependencies are statically defined, which
should be fine and is still a much better solution than
to have no dependencies.

Also delete empty example-fingerprint.rst.

(cherry picked from commit 529cc6a52fc45a1a67abb5c91819ba4fb2f631e3)
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by:
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220210104445.16767-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23747.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>