ARROW-17217: [Docs][Python] Adding pandas as required dependency

[toddfarmer]

Pandas seems to be a build requirement for documentation, but is not listed in conda or pip definitions.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-17217

[ursabot]

Benchmark runs are scheduled for baseline = 025ce54 and contender = a963392. a963392 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Failed ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Finished ⬇️0.55% ⬆️0.0%] test-mac-arm
[Finished ⬇️0.27% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.32% ⬆️0.04%] ursa-thinkcentre-m75q
Buildkite builds:
[Failed] a963392c ec2-t3-xlarge-us-east-2
[Finished] a963392c test-mac-arm
[Finished] a963392c ursa-i9-9960x
[Finished] a963392c ursa-thinkcentre-m75q
[Failed] 025ce542 ec2-t3-xlarge-us-east-2
[Finished] 025ce542 test-mac-arm
[Finished] 025ce542 ursa-i9-9960x
[Finished] 025ce542 ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java

[ursabot]

['Python', 'R'] benchmarks have high level of regressions.
test-mac-arm