update texts and remove obsolete assets, re #2

Makefile

README.md

ToolAssistant.php

assets/assistant.css

assets/assistant.js

assets/bbb-raum.jpg

assets/cheatsheet-digital-teaching.pdf

assets/cheatsheet-teach-online.pdf

assets/en/studip-courseware-aktivieren.jpg

assets/en/studip-courseware-bearbeiten.jpg

assets/en/studip-dateien.jpg

assets/en/studip-hausaufgaben-aktionen.jpg

assets/en/studip-hausaufgaben-typ.jpg

assets/en/studip-meetings-raumname.jpg

assets/en/studip-mehr.jpg

assets/en/studip-menu-courseware.jpg

assets/en/studip-menu-dateien.jpg

assets/en/studip-menu-meetings.jpg

assets/en/studip-menu-pad.jpg

assets/en/studip-menu-teilnehmende.jpg