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.

README.md

@@ -28,14 +28,18 @@ By participating, you are expected to uphold our [Code of Conduct](https://gitla
 
 * You found a bug, have a question, or want to request a feature? Please 
 [create an issue](https://gitlab.com/caosdb/caosdb-pylib/-/issues).
-* You want to contribute code? Please fork the repository and create a merge 
-request in GitLab and choose this repository as target. Make sure to select
-"Allow commits from members who can merge the target branch" under Contribution
-when creating the merge request. This allows our team to work with you on your request.
-- If you have a suggestion for the [documentation](https://docs.indiscale.com/caosdb-pylib/), 
+* You want to contribute code?
+    * **Forking:** Please fork the repository and create a merge request in GitLab and choose this repository as
+      target. Make sure to select "Allow commits from members who can merge the target branch" under
+      Contribution when creating the merge request. This allows our team to work with you on your
+      request.
+    * **Code style:** This project adhers to the PEP8 recommendations, you can test your code style
+      using the `autopep8` tool (`autopep8 -i -r ./`).  Please write your doc strings following the
+      [NumpyDoc](https://numpydoc.readthedocs.io/en/latest/format.html) conventions.
+* If you have a suggestion for the [documentation](https://docs.indiscale.com/caosdb-pylib/),
 the preferred way is also a merge request as describe above (the documentation resides in `src/doc`).
 However, you can also create an issue for it. 
-- You can also contact us at **info (AT) caosdb.de** and join the
+* You can also contact us at **info (AT) caosdb.de** and join the
   CaosDB community on
   [#caosdb:matrix.org](https://matrix.to/#/!unwwlTfOznjEnMMXxf:matrix.org).