docs: add Core v3 migration guide (FEP-2667)

[ENvironmentSet]

Summary

• add Korean and English Core v2 to v3 migration guides
• document plugin hook, initInfo, SnapshotEvent, and action mock migrations
• explain snapshot capture, restore, error policy, and Replaced event validation
• register both guides in the docs sidebar

Linear: FEP-2667

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Latest commit: dadae52

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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