[V3] Improve documentation

We fix documentation findings:

Remove references to Data_type_def from the module docstring, as it has been removed in v3.0 of the spec. Fixes #231.
Add missing spec docstring to Key.type. Fixes #242.
Expand description to AAS_referables to include its necessity to exist, since this enum does not exist in the spec. Fixes #246
Document in the module docstring that AASc-3a-003 cannot be implemented. Fixes #251.
Document in the module docstring, how Lang_string-subtypes are implemented. Fixes #262.

aas-core-works / aas-core-meta