[Doc PR Part 3] Add scripts to automatically generate API reference md files

[chenmoneygithub]

We add two scripts:

1. generate_api_docs.py: Generate an .md file for each public class/function.
2. generate_api_summary.py: Generate the index in mkdocs.yml to refer to the doc page for each public class/function.

At doc deployment time, these 2 scripts should be executed to generate the latest documentation.

Some notes:

1. Many DSPy classes/functions can be accessed by multiple path, e.g., dspy.ChainOfThought, dspy.predict.ChainOfThought, dspy.predict.chain_of_thought.ChainOfThought all point to the same thing. We only generate doc for the top level dspy.ChainOfThought, which is our recommended path
2. We only generate API references for classes/functions that are exposed through __all__ attribute in __init__.py files. Or say we only generate API reference for classes/functions we would like to have users interact with.