Add getting started guide for admins

[ralf401]

• Guide new Zammad admins through the configuration of the most important features

• Cover features which are probably useful for most of the Zammad users

• Add informative videos to quickly transport config information

• Add 'Start With Zammad' page

• Add 'Additional Configuration' page

• Add groups/roles graphic

• Create getting started wizard video

• Create groups/roles video

• Create email channel video with group assignment and signatures

• Create invite agents video

• Create text modules video

• Create object attribute video

• Create checklist video

• Create AI agents video

• Include links to new media server

• Remove video support for RTD