Add an example for creating a phenopacket in Python

[ielis]

The PR extends the documentation with an example of building a phenopacket in Python.

The new document at docs/rd-example-python.rst shows Python snippets for creating a phenopacket representing a case report about a boy with Bethlem myopathy presented in A complete example: Rare Disease.

The Python snippets are checked in the CI to ensure accuracy. Moreover, a test at python/tests/test_rd_example.py checks if such phenopacket can be built.

The PR addresses #416