Docs: Accessibility Indicators - ÖV Güteklassen

[UJehle]

### Tasks
- [x] Explanation (Ulrike)
- [x] Step-by-step guideline (Irem)
- [x] Update Screenshot in "Visualization" section (Irem)
- [x] Check step-by-step guideline (Ulrike)

[EPajares]

Here is the one for ÖV Güteklassen; https://github.com/goat-community/goat-core/blob/main/src/schemas/motorized_mobility.py

[nihar1024]

TODO: Add geofence

[AnaSasso]

done