GH-32503: [Docs][Benchmarking] Document pull request benchmark hooks

[nightcityblade]

Rationale for this change

Fixes #32503.

The developer benchmark guide explains local Archery runs but does not describe how pull requests use the remote benchmark service or repository-owned build hooks.

What changes are included in this PR?

• Document the pull request comment that schedules remote benchmarks.
• Explain how Conbench compares the contender and baseline commits.
• Link the benchmark environment and hooks documentation and clarify that hook changes are tested from the pull request commit.

Are these changes tested?

• uvx sphinx-lint docs/source/developers/benchmarks.rst
• make -C docs dev with the documented requirements installed in an isolated environment
• Inspected the rendered benchmarks page to verify the new section, command block, and link.

Are there any user-facing changes?

Yes. The developer documentation now explains pull request benchmarking and benchmark hook changes.

AI usage

This documentation update was drafted with Codex assistance. I reviewed the current workflow against recent Arrow pull request benchmark comments, checked every line of the change, and verified the rendered output locally.

• GitHub Issue: [Docs][Benchmarking] Add info about the use of benchmarking hooks on the PR #32503

[github-actions]

⚠️ GitHub issue #32503 has been automatically assigned in GitHub to PR creator.

[uros-b]

Thank you @nightcityblade! LGTM