Document synchronous entity creation and flag targeting

[roncohen]

Summary

• show direct HTTP/cURL examples for creating a user and immediately enabling a flag
• replace the legacy targeting endpoint with the current entity-specific endpoint
• distinguish synchronous entity availability from flag configuration propagation
• clarify when to use Management and Runtime APIs
• document required write:entities and write:flag:targeting scopes
• add entity PUT/DELETE reference entries and remove duplicate operations

SDK examples remain in the Management SDK documentation (reflagcom/javascript#725). Generated SDK docs are untouched.

Validation

• all 16 reference operations matched the published OpenAPI 3.1.0 spec
• relative link targets checked
• shell examples pass bash -n; no mutation requests executed
• git diff --check passed

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