docs: security-bugs: add a link to the threat-model documentation

docs: security-bugs: add a link to the threat-model documentation

Rather than make readers search for this document, just a link to it where
it is referenced.

(While I was at it, I removed the unused and unneeded _threatmodel label
from the top of threat-model.rst).

Acked-by: Willy Tarreau <w@1wt.eu>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>