Confusing docstrings in ModelBuilder Example

Notebook title: Using ModelBuilder class for deploying PyMC models
Notebook url: https://github.com/pymc-devs/pymc-examples/blob/main/examples/howto/model_builder.ipynb

Issue description

Comments and docstrings in the example (class LinearModel(ModelBuilder): ...) are confusing and seemingly outdated. In particular, model_config is not a parameter of LinearModel.build_model().

Expected output

N/A

Proposed solution

Update the example, so that it follows closely in-line examples embedded in class ModelBuilder: docstrings. Perhaps, improve both at the same time.

See also: #811

Open examples/howto/model_builder.ipynb and review the LinearModel(ModelBuilder) example, then compare its comments and docstrings with the inline examples in the ModelBuilder docstrings at pymc-extras/model_builder.py. Done means the notebook documentation is current and no longer describes model_config as a parameter of LinearModel.build_model().