docs: add external approval checkpoint example

[jw-ond]

Summary

• Adds a small external approval checkpoint example to the Human in the Loop docs.
• Shows how to construct a stable action object for a pending Haystack tool call.
• Maps checkpoint decisions (allow, require_approval, deny) to continue, pause, and block behavior using ConfirmationUIResult.

Validation

• git diff --check
• checked Markdown code fences are balanced
• verified OSuite is mentioned only once as an example provider

Notes

• This is intentionally vendor-neutral and builds on the existing ConfirmationUI / BlockingConfirmationStrategy pattern.
• Full local docs build was not run because the full repository clone repeatedly disconnected and Hatch is not installed in this environment.

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[HaystackBot]

Hi @jw-ond, thanks a lot for your contribution! 🙏

We noticed that the Contributor License Agreement (CLA) check (license/cla) hasn't passed yet, so we've temporarily moved this PR to draft and paused the review assignment.

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[HaystackBot]

Thanks for signing the CLA, @jw-ond! 🎉 This PR is now ready for review again and the reviewer has been re-assigned.

[julian-risch]

Closing this because it could be an idea for https://github.com/deepset-ai/haystack-cookbook maybe but not for the Haystack documentation.