docs: Use relative links within the web page

docs: Use relative links within the web page

Replace full/external links which point to content within
'https://libvirt.org/' with relative links so that the web page works
fully locally.

This does not change the links in 'docs/manpages' as we want the
installed man page to work from everywhere (even when the local docs are
not installed) and the generated API docs which take links from the C
source.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>