[Stream] Add ERR_UNHEALTHY error code to webhooks documentation

[tsmith512]

Add error code for unhealthy broadcast due to encoder issues and link to required settings.

[cloudflare-docs-bot]

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[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/stream/	@tsmith512, @cloudflare/product-owners, @renandincer, @third774

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Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/stream/stream-live/webhooks/	https://tsmith512-patch-1.preview.developers.cloudflare.com/stream/stream-live/webhooks/