Note mandatory KNewStuff dependency

9d90ed46 added a page that mentions
KNewStuff and includes a button you can click on to open a
KNewStuff-provided app. But it neglected to mark the KNewStuff
framework as a dependency, allowing users without that framework
installed to click on the button and get a broken UX.

BUG: 465072
FIXED-IN: 5.27


(cherry picked from commit 42f404d0)