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ARROW-10018: [CI] Disable Sphinx and API documentation build on master

I haven't had the time to investigate the build logs, but it will make the build queue pretty slow.

Closes #8198 from kszucs/ARROW-10018

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>