Revamp menu guidelines

Summary:
Revamp the pages about menubars and context menus:

• Reduce redundancy
• Update guidelines according to generally accepted best practices
• Fix minor errors

Test Plan: Deploy locally and read through it

Reviewers: fabianr, KDE Human Interface Guidelines, VDG

Reviewed By: fabianr, KDE Human Interface Guidelines, VDG

Tags: KDE Human Interface Guidelines

Differential Revision: https://phabricator.kde.org/D16827