GH-51001: [CI] Document Crossbow GitHub token maintenance

[novadusk]

Issue: #51001

Rationale for this change

The Crossbow comment bot uses CROSSBOW_GITHUB_TOKEN. Contributors need to know its required scopes and the coordinated replacement steps so token rotation does not break the bot or CI.

What changes are included in this PR?

• Document the required GitHub token scopes.
• Explain when to rotate the token and that Apache INFRA updates the GitHub Actions secret.
• Document the additional updates required in the Crossbow repository and Azure Pipelines.
• Keep the guidance aligned with the existing token-expiration template.

Are these changes tested?

• Checked the instructions against the existing Crossbow workflow and token-expiration template.
• git diff --check.
• Documentation-only change; no software tests apply.

Are there any user-facing changes?

Yes. This updates contributor-facing CI documentation; it does not change runtime behavior or public APIs.

Was AI used for this PR?

AI assistance was used for repository search and drafting. I reviewed the documentation change against the existing workflow and token-expiration template.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [CI][Dev] Document how to maintain CROSSBOW_GITHUB_TOKEN #51001

[kou]

Could you use our PR template?

[github-actions]

⚠️ GitHub issue #51001 has been automatically assigned in GitHub to PR creator.