DOC: Fix and extend documentation on inheritance

Summary

See #53 (closed). No new code, just docstrings and documentation

Focus

Updated/added the docstrings of the add_parent methods of Entity, Property, and RecordType in caosdb.models. Also updated the section on inheritance in the data-insertion tutorial. If you are content with the docstrings, we can use the one of Entity.add_parent as an example for a docstring with a list of **kwargs, and the one of RecordType.add_parent as an example for linking to other functions, classes, or methods.

Test Environment

Run make doc and check the documentation. You may have to delete the _apidocs directory and re-install this pylib branch beforehand.

Check List for the Author

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• All automated tests pass
• Reference related Issues
• Up-to-date CHANGELOG.md
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there spezifications? Are they satisfied?

For further good practices have a look at our review guidelines.