Update documentation to match latest development

[mariobehling]

Checkin.md

• The first step "Select Server" is being changed. When this is complete please adjust the documentation.
• Scan QR code - Show a screenshot with cam box and QR code
• You sometimes spell "checkin" as check-in or checkin. Always use the same spelling. I recommend to stick to checkin.

Exhibitor.md

• The first step "Select Server" is being changed. When this is complete please adjust the documentation.
• Show actual cam box and QR code in the screenshot of "Welcome Exhibitor"
• On the exhibitor page, e.g. Nvidia, show the actual cam box with QR code