Add information on how to subscribe to the mailing list mentioned on "Security in Qt"

The page https://doc.qt.io/qt-6/security.html mentions in the section "Qt Project Security Policy" the use of the mailing list announce@qt-project.org , but does not refer to a page which explains how to subscribe for this list. A user needs to google or ask an AI to search. IMHO, it is a far better user expereince to mention this essential information on this page or refer to another page which does this.