GH-41620: [Docs] Document merge.conf usage

[danepitkin]

Rationale for this change

As a new committer, I found that the usage of merge.conf was not documented and that a placeholder Jira token is still required, even though Arrow no longer uses Jira.

What changes are included in this PR?

• Document merge.conf usage

Are these changes tested?

n/a

Are there any user-facing changes?

No

• GitHub Issue: [Docs] Add documentation for merge.conf #41620

[github-actions]

⚠️ GitHub issue #41620 has been automatically assigned in GitHub to PR creator.

[danepitkin]

Do we still use this script for merging Parquet Jira issues?

[assignUser]

Do we still use this script for merging Parquet Jira issues?

A single one was merged in 16.0.0. There is an ongoing/pending thread on the parquet-ml https://lists.apache.org/thread/wvbg0tgx81xyxn5v2dgxt7b8l8o2sv4x
But there was vote called for now. I will bump the thread.

[danepitkin]

I will leave Jira token scripts alone for now!

[assignUser]

Thanks!

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 5 benchmarking runs that have been run so far on merge-commit 8d687b0.

There was 1 benchmark result indicating a performance regression:

• Commit Run on ec2-t3-xlarge-us-east-2 at 2024-05-17 18:08:54Z
  • file-read (R) with compression=lz4, dataset=nyctaxi_2010-01, file_type=feather, language=R, output_type=table

The full Conbench report has more details. It also includes information about 1 possible false positive for unstable benchmarks that are known to sometimes produce them.