docs: Provide a list of available observations

[tukusejssirs]

In the docs, you simply state that the MQTT client sends observations in fanuc/{machine-id}/{observation-name} topics, however, you don’t provide a list of {observation-name} values.

If you provide the list elsewhere in the docs, I suggest to add a link to that list in here.

[MRIIOT]

Agreed. Observations are all the veneers. https://github.com/Ladder99/fanuc-driver/blob/main/fanuc/veneers/GCodeBlocks.cs being a good example of an observation.