docs(sync): document foreign key resolvers

[Soner (shyim)]

Summary

Adds a new Foreign key resolvers section to the Sync API guide (docs/concepts/endpoint-structure/writing-entities/bulk-payloads.md).

The Sync API has supported FK resolvers for a while via ProductNumberFkResolver, but the feature has never been documented. This PR fills that gap and lists the seven new tier-1 resolvers landing in the platform repo:

• product.number (existing)
• currency.iso_code
• locale.code
• payment_method.technical_name
• shipping_method.technical_name
• document_type.technical_name
• salutation.salutation_key
• tax.tax_rate

The new section explains the { resolver, value, nullOnMissing } payload shape, lists each built-in resolver with the column it looks up, includes a runnable example using currency.iso_code and tax.tax_rate together, and warns explicitly that tax.tax_rate skips ambiguous rates instead of picking one — a behavior integrators will otherwise hit blind.

Draft until the platform PR for the new resolvers merges, since the resolver names need to match.

Test plan

• Render the page locally and confirm the table and example code blocks display correctly
• Cross-check resolver names against the platform PR once merged
• Verify the warning theme block renders as a callout (matches the existing theme: warning usage in README.md)

[somethings (lasomethingsomething)]

cc Patryk Tomczyk (@patzick) Michael Telgmann (@mitelg) Björn Meyer (@BrocksiNet) Martin Bens (@SpiGAndromeda)

[Patryk Tomczyk (patzick)]

Would be nice to update OpenAPI schema with this knowledge, currently it defines pauload as array of objects, so basically anything

"payload": {
                "description": "Contains a list of changesets for an entity. If the action type is `delete`,\n    a list of identifiers can be provided.",
                "type": "array",
                "items": {
                  "type": "object"
                }
              },

current preview of the schema