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ARROW-7834: [Release] Post release task for updating the documentations

Generate prose and apidocs and update arrow-site's asf-site branch as a post-release task.

Closes #6407 from kszucs/release-docs and squashes the following commits:

bfb9c546a <Krisztián Szűcs> fix review comments
b34729f07 <Krisztián Szűcs> post release script for updating docs

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