Add Async migrations docs

[Lior539]

As mentioned in this comment from another PR, Im adding the old docs on Async Migration from the PostHog public runbook into our PostHog repo:

PostHog/posthog.com#6796 remove the Self hosting runbook + Helm references from the public docs.

In the linked issue, @tiina303 PostHog/posthog.com#6791 (comment) that some of the docs may still be useful. Hence this PR adds them to the runbook.

From @xvello:

The only things to salvage are:
the async migrations, but https://github.com/PostHog/posthog/tree/master/posthog/async_migrations would be a better home for that

[posthog-bot]

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[tiina303]

Closing this per https://posthog.slack.com/archives/C0374DA782U/p1702051017569959