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