ARROW-14189: [Docs] Add version dropdown to the sphinx docs

[jorisvandenbossche]

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https://issues.apache.org/jira/browse/ARROW-14189

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[jorisvandenbossche]

There is working happening for this in the upstream theme (pydata/pydata-sphinx-theme#436), but since that is not yet merged/released, I will "backport" it to our own docs.

The basics already work here:

Some notes:

• This is the basic, default styling of a dropdown from bootstrap (using the bootstrap primary color). We might want to style it a bit differently to have it fit better with our color scheme
• Exact placement of the dropdown on the page can be improved
• You can go back to earlier versions (and it preserves the page you are on), but those earlier versions don't have this dropdown (unless we would rebuild the older docs with this patch).

[kszucs]

Thanks @jorisvandenbossche for working on this!

We need to update the release script to create and mount the right directory to the docker container which builds the documentation:

mkdir -p ${ARROW_SITE_DIR}/docs/${version}
# ...
-v "${ARROW_SITE_DIR}/docs/${version}:/build/docs"

We either need to use full version numbers in the selector or trim the version number in the release script to major and minor parts.

Other than that can we merge this PR?

[kszucs]

Thanks Joris!

[ursabot]

Benchmark runs are scheduled for baseline = 311a953 and contender = 9c57eb3. 9c57eb3 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Finished ⬇️1.03% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️1.07% ⬆️0.58%] ursa-thinkcentre-m75q
Supported benchmarks:
ursa-i9-9960x: langs = Python, R, JavaScript
ursa-thinkcentre-m75q: langs = C++, Java
ec2-t3-xlarge-us-east-2: cloud = True