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ARROW-7054: [Docs] Enable overriding project version with environment variable when building Sphinx docs

This is a workaround for ARROW-7053, but it can also help with adding new sections to the docs in between releases when needed.

Closes #5766 from wesm/ARROW-7054 and squashes the following commits:

b2254c264 <Wes McKinney> Remove 'spoofed'
a1d114988 <Wes McKinney> Enable spoofing project version when building Sphinx docs

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>