[SYSTEMDS-3330] Documentation of builtin functions

[anuunchin]

Description

This PR resolves issue 3303 and is a part of the AMLS course.

Overview (as of now)

To resolve the issue, the following tasks are to be carried out:

• Fix the warnings and errors currently thrown by the src/main/python/generator/generator.py.
• Starting with the following builtin functions, add a python and dml example in a format of a code block, but under a different tag:
  • dist.dml
  • lm.dml
  • normalize.dml
  • toOneHot.dml
  • randomForest.dml
  • img_*.dml
• Adjust src/main/python/generator/generator.py and src/main/python/generator/dml_parser.py accordingly to handle the python examples and transform them into test cases, possibly using a new script under src/main/python/generator/test_generator.py.

[Baunsgaard]

I have a few early comments.

[anuunchin]

This PR is separated into a prep PR #2291 that resolves existing issues thrown by generator.py and the main PR #2292.