The previously added AMD SEV doc was not linked from anywhere on the
website. Address this by introducing a new "Knowledge base" section
that can hold task oriented guide to various features. Moving the SEV,
disk locking and secure usage guides under this section.
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>