ARROW-7561: [Doc][Python] Add missing conda_env_gandiva.yml in python.rst
ARROW-7561: [Doc][Python] Add missing conda_env_gandiva.yml in python.rst

In the python/development.html page add the missing "--file conda_env_gandiva.yml" argument to the conda setup command.

Closes #6178 from maartenb/doc-fix and squashes the following commits:

4b63bc19b <Maarten Ballintijn> Add test data setup
b7996a9cc <Maarten Ballintijn> Add missing conda_env_gandiva.yml

Authored-by: Maarten Ballintijn <3d56b4b950b1568b77f891740813584dd441f246@xs4all.nl>
Signed-off-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>

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