Add informative text and an example for relative tm:ref

[JKRhb]

As discussed in #1546, this PR adds a few informative sentences regarding relative tm:ref imports, as well as an additional example (which could probably be simplified).

Resolves #1546

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Preview | Diff

[thjaeckle]

The JSON intendation of the example could be improved.
Apart from that: Looks good to me.

[sebastiankb]

Thanks, looks good to me too. There could be more text about the example, however, in my view it's simple and kind of self-explanatory.

[sebastiankb]

from today's TD call:

• decided to merge
• another issue will be created that we need another statement if the non-manditory properties should be derived into a TD instance or not (e.g., "genericTemperature")