fix: add health check to exporting logs documentation

[rolznz]

Summary of changes

The log shipper sometimes stops working randomly, so it's good to have a health check. This configuration was recommended to me by Leslie from Fly support.

Preview

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Related Fly.io community and GitHub links

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Notes

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[kcmartin]

Sorry for the long silence on this, and thanks for flagging the silent-failure problem. Since this was opened we've reworked the page in #2420 to remove the public service entirely (it exposed Vector's API publicly), so this diff no longer applies. We've added a health check section there instead, using a top-level [checks] block with your check settings, which works without a public service. Closing in favor of #2420. Thanks again for the contribution!