ci: add stale PR management workflow

[AR21SM]

Description of Changes

Add GitHub Actions workflow to automatically manage inactive pull requests:

• Marks PRs as stale after 30 days of inactivity
• Closes stale PRs after 14 additional days without activity
• Exempts draft PRs and those with specific labels or milestones
• Configures proper permissions and runs daily

Related Issue

closes #346

Type of Change

Please check the appropriate box that describes your PR:

• 🐛 Bug Fix — Fixes a documented issue or incorrect behavior
• ✨ Content Enhancement — Updates or improves existing content
• ✍️ New Content — Adds new chapters, examples, or significant sections
• 🛠️ Other — Please describe in the Description of Changes section

Additional Notes (Optional)

Checklist

• I have searched open pull requests to avoid duplicates.
• I have built the book locally using the build instructions and verified my changes work as expected.
• I have written clear, conventional commit messages, as per the Pull Request Guidelines.

[AR21SM]

Hi @aterrel!
I've set up a stale PR workflow. These labels will exempt PRs from being marked stale:
https://github.com/actions/stale#:~:text=exempt%2Dpr%2Dlabels,exempted%20from%20stale

🔄 changes required
✨ content enhancement
✍️ new content

Let me know if you want to add any other exempt labels.

[aterrel]

I think we go ahead and get this in once we fix the closing issues rather than just marking them stale.

What would be great is if there is a new PR for an issue, that owner is assigned the issue so that other contributors know someone is working on the issue.

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