Address usage mistakes in
https://github.com/lancedb/lancedb/issues/2135.
* Add example of how to use `LanceModel` and `Vector` decorator
* Add test for pydantic doc
* Fix the example to directly use LanceModel instead of calling
`MyModel.to_arrow_schema()` in the example.
* Add cross-reference link to pydantic doc site
* Configure mkdocs to watch code changes in python directory.
