Added section about switching tabs with keyboard shortcuts to tab page

Summary: Added section to switch between tabs with ctrl + ⇥

Reviewers: KDE Human Interface Guidelines, ngraham

Reviewed By: KDE Human Interface Guidelines, ngraham

Subscribers: ngraham

Tags: KDE Human Interface Guidelines

Maniphest Tasks: T10233

Differential Revision: https://phabricator.kde.org/D17821