documentation: make section levels consistent

documentation: make section levels consistent

Previously the sections "Encryption Options" and
"Data channel cipher negotiation" were on the same
level as "OPTIONS", which makes no sense. Instead
move them and their subsections one level down.

Use ` since that was already in use in section
"Virtual Routing and Forwarding".

Change-Id: Ib5a7f9a978bda5ad58830e43580232660401f66d
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20240325071520.12513-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28453.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 3fdf5aa04f7b96a3b7110f75306306ac5d7ed5fd)