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GH-41620: [Docs] Document merge.conf usage (#41621)

### 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: #41620

Authored-by: Dane Pitkin <dpitkin@apache.org>
Signed-off-by: Dane Pitkin <dpitkin@apache.org>