docs: party role relationships on tracking request creation

[maurycy]

Docs for DEV-12295. Draft until the API change ships; merge together with it.

What

• openapi.json: POST /tracking_requests accepts shipper, consignee, notify_party, customs_broker, freight_forwarder, and pickup_dray_carrier relationships next to customer, one party per role. Example request updated, 422 example for a party from another account.
• Guide Assign Shipper, Consignee, and Other Parties: creation-time assignment becomes the first path, the shipment endpoints stay for existing shipments and for multiple parties in one role.
• adding-customer guide: one sentence pointing at the other roles.
• Changelog entry labelled Unreleased. Set the date at merge.

Contract as proposed in DEV-12295

• Relationship per role, data is a single party.
• Roles are written as PartyRole on the tracking request and copied to the shipment by the existing propagation.
• Party from another account: 422 with source.pointer on the relationship, whole request rolled back.
• Tracking request response unchanged; roles are read on the shipment.

If the implementation departs from this, update the docs here before merging.

🤖 Generated with Claude Code

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DEV-12295