Issue with wheels including docs and examples folder

[ethanluoyc]

Creating wheels from the git repository includes the docs and examples folder.

python -m build .

unzip -t dist/dm_haiku-0.0.11.dev0-py3-none-any.whl
Archive:  dist/dm_haiku-0.0.11.dev0-py3-none-any.whl
    testing: docs/conf.py             OK
    testing: docs/ext/coverage_check.py   OK
    testing: examples/imagenet/__init__.py   OK
    testing: examples/imagenet/dataset.py   OK
    testing: examples/imagenet/train.py   OK
    testing: examples/impala/__init__.py   OK
    testing: examples/impala/actor.py   OK
    testing: examples/impala/actor_test.py   OK
    testing: examples/impala/agent.py   OK
    testing: examples/impala/haiku_nets.py   OK
    testing: examples/impala/learner.py   OK
    testing: examples/impala/learner_test.py   OK
    testing: examples/impala/run_catch.py   OK
    testing: examples/impala/util.py   OK
    testing: examples/rnn/__init__.py   OK
    testing: examples/rnn/dataset.py   OK
    testing: examples/rnn/train.py    OK
    testing: examples/transformer/__init__.py   OK
    testing: examples/transformer/dataset.py   OK
    testing: examples/transformer/model.py   OK
    testing: examples/transformer/train.py   OK

Similar issue in rlax google-deepmind/rlax#133