feat: add new docs for DKIM Key endpoints and options TASK-169563

[rosadajake]

Pull Request Summary

DKIM Keys API Documentation

Added comprehensive documentation for the new DKIM Keys API endpoints, allowing customers to manage multiple DKIM keys per sending domain.

Changes to content/api/sending-domains.apib

Added a new DKIM Keys section including:

• DKIM Key Object - Field definitions and sample JSON
• DKIM Key Constraints - Limits table (max 256 keys per domain, ID format rules, etc.)
• DNS Verification Requirements - TXT record format and verification requirements
• DKIM Key Error Codes - Comprehensive error codes table with HTTP statuses and descriptions
• 6 new endpoints:
  • GET /v1/sending-domains/{domain}/dkim-keys - List all DKIM keys
  • POST /v1/sending-domains/{domain}/dkim-keys - Create a DKIM key (auto-generate or user-provided)
  • GET /v1/sending-domains/{domain}/dkim-keys/{key_id} - Retrieve a specific DKIM key
  • PUT /v1/sending-domains/{domain}/dkim-keys/{key_id} - Update a DKIM key
  • DELETE /v1/sending-domains/{domain}/dkim-keys/{key_id} - Delete a DKIM key
  • POST /v1/sending-domains/{domain}/dkim-keys/{key_id}/verify - Verify a DKIM key's DNS record

Changes to content/api/transmissions.apib

• Added options.dkim_key field to select which DKIM key to use for signing
• Added error responses for invalid/unverified DKIM key scenarios
• Updated sample JSON to include dkim_key example

Changes to content/api/smtp.apib

• Added options.dkim_key field to the X-MSYS-API header options
• Updated X-MSYS-API example JSON to include dkim_key

---

Note

Introduces DKIM key management docs and wiring to use specific keys in sends.

• New DKIM Keys section in sending-domains.apib: object schema, constraints, DNS requirements, error codes, and endpoints: GET /v1/sending-domains/{domain}/dkim-keys, POST /.../dkim-keys, GET /.../dkim-keys/{key_id}, PUT /.../dkim-keys/{key_id}, DELETE /.../dkim-keys/{key_id}, POST /.../dkim-keys/{key_id}/verify
• Transmissions: add options.dkim_key to select a verified key; update sample; add 400 errors for invalid/unverified key
• SMTP: add options.dkim_key in X-MSYS-API options and example
• Sending Domains: remove Enterprise-only qualifier from dkim.signing_domain in create/update docs

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[rafaelmatsumotomb]

For the regular sending domain we only allow custom signing_domain for Enterprise customer, we should doublecheck if we want to keep it like that