docs: add comprehensive interface mapping and fix migration guide

[ryanbas21]

JIRA Ticket

https://pingidentity.atlassian.net/browse/SDKS-4884

Description

Add interface_mapping.md — exhaustive API-level mapping of all 81 legacy @forgerock/javascript-sdk exports to their new Ping SDK equivalents. Designed for both developer reference and AI-assisted refactoring.

Update MIGRATION.md — correct wrong import paths (oidc from wrong package), fix .well-known URL typo, fix incorrect tree-passing documentation, add missing packages/methods/step types, and link to interface_mapping.md.

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