docs: explain external user lookup for Identity and CMS Kit

[enisn]

Summary

• document how the Identity module resolves external users and when IIdentityUserIntegrationService is used
• add CMS Kit guidance for tiered and distributed setups that need remote user lookup with client credentials
• clarify the current integration service entry point in the Identity application services section

Testing

• git diff --check

Closes #14079

[Copilot]

Pull request overview

Adds documentation explaining how ABP resolves “external” users for module-to-module scenarios, and how to configure remote (tiered/distributed) user lookup between CMS Kit and Identity using Identity integration services.

Changes:

• Document UserLookupService external lookup flow in the Identity module docs, including the relevant provider implementations and remote setup prerequisites.
• Add CMS Kit guidance for remote user lookup against Identity in tiered/distributed deployments (RemoteServices + client credentials + exposed integration services).
• Clarify Identity’s current integration-service entry point by documenting IdentityUserIntegrationService and noting IdentityUserLookupAppService’s backward-compat delegation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/en/modules/identity.md	Adds an “External User Lookup Service” section and updates the application-services list to highlight the integration service entry point.
docs/en/modules/cms-kit/index.md	Adds a new section describing when/how CMS Kit performs user lookups via Identity integration endpoints in distributed setups.