Create an early migration guide for the new start_span API #5318
Description
Since span first will use a new start_span API, we'll need to migrate custom instrumentation in Seer to it before we can actually use the new SDK on it.
Prepare a migration guide that someone else can follow to start the Seer migration early (and that can then also serve as a migration guide for folks once we release span first for real).
Migration guide: getsentry/sentry-docs#16072