Write display_configuration

[balk77]

Write https://github.com/balk77/balk77.github.io/blob/main/docs/display_configuration.md

Suggestions for content:

• General concept, explaining eventtypes
• Pages concept (as of firmware 1.09)
• Value, and its MQTT topic (with example)
• Label, and its MQTT topic (with example)
• Unit, and its MQTT topic (with example)
• Alignment
• x/y offset
• Fontsize
• Width
• Rounding
• Brightness
• Statusbar