Add API documentation for account ownership endpoints

[robzolkos]

Summary

Document the JSON account ownership management endpoints added in https://github.com/basecamp/bc3/pull/10043

Changes

• Add docs for POST /account/people/:person_id/ownership.json
• Add docs for DELETE /account/people/:person_id/ownership.json
• Note the account-owner-only permission requirements
• Document the returned Person JSON and owner flag behavior
• Add cURL examples for both endpoints

[Copilot]

Pull request overview

This PR adds API documentation for two account ownership management endpoints that were previously implemented. The documentation follows the established pattern used throughout the API documentation, providing clear descriptions of each endpoint's purpose, parameters, return values, and cURL examples.

Changes:

• Add documentation for POST endpoint to grant account ownership to a person
• Add documentation for DELETE endpoint to revoke account ownership from a person
• Add reference link to the Person resource documentation

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