Define or describe possible values for field `standard-compliance`

[hoijui]

It is currently underdefined.
It is simply a free form text value. This means, people could write stuff like:

• "DIN SPEC"
• "DIN SPEC 3105"
• "DIN SPEC 3105-1"
• "3105-1"
• "3105"

... and mean the same thing.
That makes this field basically non-machine-readable, and thus quite useless for common use-cases of OKH.