add test on docs in the package

[jameslamb]

In pandas-dev/pandas#38846, I'm proposing that the contents of doc/ be removed from package artifacts. See the discussion there and in pandas-dev/pandas#30741 for more background.

During review on that PR, I was asked to include a test here in pandas-release to check that docs haven't been inadvertently included in artifacts (pandas-dev/pandas#38846 (comment)). This PR proposes that test.

If you run the following

make init-repos
make docker-image
make pip-test

The build will fail with this error

Files from doc/ should not be included in the package, but 223 were found.
first few files:

doc/README.rst
doc/make.py
doc/_templates/autosummary/accessor.rst
doc/_templates/autosummary/accessor_attribute.rst
doc/_templates/autosummary/accessor_callable.rst
doc/_templates/autosummary/accessor_method.rst
doc/_templates/autosummary/class.rst
doc/_templates/autosummary/class_without_autosummary.rst
doc/cheatsheet/Pandas_Cheat_Sheet.pdf
doc/cheatsheet/Pandas_Cheat_Sheet.pptx
doc/cheatsheet/Pandas_Cheat_Sheet_JA.pdf
doc/cheatsheet/Pandas_Cheat_Sheet_JA.pptx
doc/cheatsheet/README.txt
doc/data/fx_prices
doc/data/iris.data
doc/source/conf.py
doc/source/ecosystem.rst
doc/source/index.rst.template
doc/source/_static/index_api.svg
doc/source/_static/index_contribute.svg

Makefile:53: recipe for target 'pandas/dist/pandas-1.1.0.tar.gz' failed
make: *** [pandas/dist/pandas-1.1.0.tar.gz] Error 1

This is my first time in this repo, so if I didn't put this test in the right place please let me know.

Thanks!

[jameslamb]

I opened this at the request of maintainers in pandas-dev/pandas#38846, but it seems like it might not be something the maintainers here are interested in. I'm going to close this, please @ me in the future if you'd like me to add it back.

[jbrockmendel]

cc @simonjayhawkins this might have slipped through the cracks?