[CI][Documentation] Disable Unity builds to fix OpenTelemetry symbol collision in docs build

[HyukjinKwon]

Describe the bug, including details regarding any error messages, version, and platform.

https://github.com/apache/arrow/actions/runs/20768594219/job/59639781637

Component(s)

Documentation

[raulcd]

Issue resolved by pull request 48751
#48751