Document a basic operational level agreement for Durable Sync

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We don't have one, and it would be useful to have something moving forward.

Based on the guidelines provided by @Micheletto here, let's put together a draft of what this could look like.

I'll draft this, then we can review until Ops & Eng teams feel comfortable moving forward.

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Doc lives here

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Related bugzilla issue to get dashboard created for monitoring.