remove docs from packages
- ~closes #xxxx~
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

#30741 discusses removing documentation and tests from package distributions. This is valuable because it makes `pandas` easier to use in storage-sensitive environments such as AWS Lambda (https://github.com/pandas-dev/pandas/issues/30741#issuecomment-638158062, https://github.com/pandas-dev/pandas/issues/30741#issuecomment-752751976).

This PR does not close that issue, but it's a first step. This proposes removing the content of `doc/`  from package distributions. This PR trims about 1MB (compressed) and 6MB (uncompressed) out of the `sdist` package.

|                                        |  `master`  | this PR   |
|:-----------------------:|:----------:|:--------:|
| sdist (compressed)       |     5.7M      |  4.5M     |
| sdist (uncompressed)   |     25M       |  19M      |

<details><summary>how I checked these sizes</summary>

```shell
rm -rf pandas.egg-info
rm -rf dist/
rm -rf __pycache__

echo ""
echo "building source distribution"
echo ""
python setup.py sdist > /dev/null
cp pandas.egg-info/SOURCES.txt ~/PANDAS-SOURCES.txt
pushd dist/
    echo ""
    echo "sdist compressed size"
    echo ""
    du -a -h .
    tar -xf pandas*.tar.gz
    rm pandas*.tar.gz
    ls .
    echo ""
    echo "sdist uncompressed size"
    echo ""
    du -sh .
popd
```

</details>

To confirm that the documentation files were removed correctly by these changes, I ran the following

```shell
python setup.py sdist
cat pandas.egg-info/SOURCES.txt | grep -E "^doc"
```

Thanks for your time and consideration.