Draft: kcms/feedback: add missing help page button

Draft because I can't actually get the button to show up yet; can someone tell me what I'm missing to fix this?

The docbook exists and can be found in the global help app, but not directly from the KCM. The button doesn't show up in the UI at all rather than just linking to a missing page, so I think some other configuration missing QML/C++ wise? The source doc is in the plasma-desktop repo, I adjusted the path to what is specified in the CMake file there.

I know there is work ongoing to remove this KCM entirely, but until that actually happens let's make use of the documentation we have, since it's not exactly the most self-explanatory settings module.