Add "Show intro page" button to System Settings sidebar
Summary:
BUG: 405956
Test Plan: Open System Settings and click on the "Show intro page" button.
Reviewers: Plasma, VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: ngraham, mart, VDG, plasma-devel, Plasma
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D23274