docs(sending-domains): add dkim_key_length to parent object schema and create response

[seewerah]

Summary

• Add the dkim_key_length field to the Sending Domain Object's dkim sub-object definition, matching what was already documented on the DKIM Keys sub-resource
• Add "dkim_key_length": 2048 to the POST create response example

Companion to SparkPost/Momentum#1241, which exposes dkim_key_length on the parent sending-domain create response and (in a follow-up) on GET/List responses.

Test plan

• Verify the rendered docs show dkim_key_length in the Sending Domain Object schema
• Verify the POST create response example includes "dkim_key_length": 2048

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Note

Low Risk
Low risk docs-only change that updates the Sending Domains API schema/examples without affecting runtime behavior.

Overview
Adds dkim_key_length to the Sending Domain object’s dkim sub-object schema to document the RSA key size derived from the current public key.

Updates the POST /v1/sending-domains success response example to include dkim_key_length in the returned dkim payload.

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