Fix setup.py packages to avoid installing "examples" and "docs"

[natsukium]

This project has unintentionally included docs and examples in the packages.
As a result, running pip install dm-haiku will include them under site-packages, and you can import them like from docs import conf.

It is a common pitfall when using setuptools.find_namespace_packages() with flat layouts, and you need to set the excludes appropriately.

Steps To Reproduce

1. Prepare a clean Python environment with docker, venv, etc.
   docker run --rm -it python:3.10-slim bash
2. Install dm-haiku
   pip install dm-haiku
3. Check under site-packages
   ls /usr/local/lib/python3.10/site-packages/ | grep docs
4. Check if the docs can be imported
   python -c 'from docs import conf'