docs(sending-domains): default new DKIM keys to 2048 bits (SparkPost/Momentum#1241)

[seewerah]

Summary

• Update the dkim_key_length parameter on POST /v1/sending-domains and on POST /v1/sending-domains/{domain}/dkim-keys to document the new 2048-bit default and frame the 1024-bit value as an opt-in escape hatch for DNS providers that can't return multi-string TXT records.
• Reframe the existing TXT-record-length note around the new default: 2048-bit public keys produce TXT values ~400 characters long, which exceed the 255-character single-string TXT limit and must therefore be returned as multiple quoted strings; mainstream managed-DNS providers (Route 53, Cloudflare, Google Cloud DNS, Azure DNS) do this split automatically.
• Document the new dkim_key_length field that now appears on every DKIM-key read path (POST create, single GET, list GET, PUT update) in SparkPost/Momentum#1241: add it to the DKIM Key Object schema, the sample payload, the DKIM Key Constraints table (new "Default key length: 2048 bits" row), and every response example for the dkim-keys sub-resource so the docs match what the API actually returns.

Why

Matches the behavior change shipping in SparkPost/Momentum#1241. Without these doc updates, customers reading the page would still see "Default: 1024" and would be surprised by a TXT record longer than the docs prepare them for, and the new response field would be undocumented. 2048-bit RSA is the modern DKIM baseline (accepted/preferred by Gmail, Microsoft, Yahoo) and matches what mcp-server-sparkpost already does for newly created domains.

Scope notes

• The Momentum PR exposes dkim_key_length on the dkim-keys sub-resource responses only. The dkim sub-object returned by the parent POST /sending-domains response was intentionally left as-is in Momentum, so the parent-endpoint response examples here are not updated to include dkim_key_length. If we later decide to surface it on the parent endpoint's dkim sub-object too, a follow-up doc PR can cover those samples.
• This PR is docs-only. No code paths in this repo are touched.

Test plan

• Render the Sending Domains page locally (or in Netlify preview) and spot-check that:
  • The dkim_key_length parameter blocks on both create endpoints render with "Default: 2048" and the rewritten DNS note.
  • The DKIM Key Object schema, sample payload, and Constraints table all show dkim_key_length.
  • Every dkim-keys response example (list / create-auto / create-user-keys / single GET / PUT header-only / PUT full replacement) shows "dkim_key_length": 2048.
• Confirm no Markdown/apib parse warnings in the build.
• (Optional) Diff render with main to confirm parent-endpoint examples are untouched.