docs: document externalId-based user methods and verify cross-SDK parity

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Summary

A developer integrating user provisioning reported that the Node SDK's externalId-based user methods were not discoverable: createUserAndMembership() / updateUser() accepted an externalId in their typed options but silently dropped it, and there was no documented way to look a user up by external ID. The SDK fix shipped in @scalekit-sdk/node 2.6.2, which now exposes getUserByExternalId, updateUserByExternalId, and deleteUserByExternalId. The documentation has not caught up.

Why this matters

externalId is the documented mechanism for cross-system user correlation. Without docs for the external-ID methods, developers fall back to ID-only lookups or assume the capability is missing, and silent drops lead to records persisted with an empty external_id.

Proposed work

• Document the externalId-based user methods (getUserByExternalId, updateUserByExternalId, deleteUserByExternalId) in the user-management reference and relevant guides.
• Document the uniqueness semantics of external_id within an organization (does a duplicate conflict, or are duplicates allowed?).
• Verify and document parity across the Python, Go, and Java SDKs — confirm equivalent external-ID methods exist, or note the gap explicitly.
• Add a short note that externalId must be forwarded on create/update so it is not silently ignored.

Source

Surfaced through developer support during implementation work. The SDK code fix is already released; this issue tracks the documentation follow-up.