Add v13 migration guide

[dtroupe-plaid]

Summary

• expand the v13 migration guide for v11/v12 integrations
• document session-based APIs, Expo Modules setup, callback payload changes, Layer, Headless, Embedded Search, FinanceKit, and SIIP -> SIPP
• align LinkEventMetadata with native callback payloads by renaming metadata_json to metadataJson
• add a README link to the v13 migration guide

Validation

• PATH=/Users/dtroupe/.nvm/versions/node/v20.19.4/bin:$PATH npm test -- --coverage --no-watch --passWithNoTests
• PATH=/Users/dtroupe/.nvm/versions/node/v20.19.4/bin:$PATH npm run build
• git diff --check

[github-actions]

Coverage after merging codex/v13-migration-guide into develop-v13 will be

100.00%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src
ReactNativePlaidLinkSdkModule.ts	100%	100%	100%	100%
index.ts	100%	100%	100%	100%