Data migrations docs should all suggest historical lane

[tiina303]

Summary

There are several different docs for data migrations / how to send initial data to PostHog. Every single one of those should use very explicitly call out "historical_migration": true, that's documented in https://posthog.com/docs/api/post-only-endpoints#batch-events

I would do the update myself except this problem: https://posthog.slack.com/archives/C01FHN8DNN6/p1710872244711039

While were at it, I'd love if we reduced the amount of changes needed to the docs in the future, e.g. we have a single file in the repo that talks about how events should be sent that's included in other custom docs.

explain what's needed and link to additional context

What kind of request is this?

• [ ] New docs page / feature
• [x] Update to existing page

Which product is this for?

• [x] Product OS (e.g. platform features)
• [x] Product analytics
• [ ] Session replay
• [ ] Feature flags
• [ ] A/B testing
• [ ] Surveys
• [x] CDP
• [ ] Data warehouse
• [ ] Other (e.g. open source specific)

What's the priority?

• [x] Urgent AND important ("great scott, this is horribly wrong / out of date")
• [ ] Important, but not urgent ("we need to do this, but not immediately")
• [ ] Non-urgent (this could be better / would be nice to have)

Because if people start sending us lots of data without the historical flag they might kill our normal ingestion. We should also say that historical ingestion is only free if that flag is used (& then make billing do that).

Is there a deadline for this change?

• [ ] Yes (please specify)
• [ ] No