doc: HTTPS upgrades and URL fixes throughout the tree

doc: HTTPS upgrades and URL fixes throughout the tree

* HTTPS upgrades
* 404 fixes, with hopefully better helpful links to the relevant documentation
* some trailing white space fixes
* resurrect utun-demo.c from a different source
* Don't touch openvpn.doxyfile.in though, as it was autogenerated
* Don't touch COPYING as it's an external license file
* The openvpn.net URLs will be addressed some other time

Signed-off-by: Christian Kujau <github@nerdbynature.de>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20251006144249.23672-3-lists@nerdbynature.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59242866/
Signed-off-by: Gert Doering <gert@greenie.muc.de>