[NRL-2049] Add link to DOS onboarding guidance alongside existing onboarding support link

Adds a hyperlink to the Digital Onboarding Service (DOS) onboarding page within the API specification onboarding section.

The new link is included alongside the existing "Onboarding support information" link to provide users with direct access to formal DOS onboarding guidance, including stages, requirements, and process expectations.

This change improves visibility of onboarding guidance without altering or removing existing content, ensuring consistency with current documentation while enhancing usability for suppliers.

Author: CHSNW6

specification/record-locator/producer.yaml

@@ -230,7 +230,7 @@ info:
 
     As part of this process, you need to demonstrate that you can manage risks and that your software conforms technically with the requirements for this API.
 
-    Information on this page might impact the design of your software. For details, see [Onboarding support information](https://digital.nhs.uk/developer/api-catalogue/national-record-locator-producer-fhir/onboarding-support-information).
+    Information on this page might impact the design of your software. For details, see [Onboarding support information](https://digital.nhs.uk/developer/api-catalogue/national-record-locator-producer-fhir/onboarding-support-information) or the [Digital Onboarding Service (DOS) onboarding page](https://digital.nhs.uk/services/national-record-locator/nrl-onboarding-guidance)
 
     To understand how our online digital onboarding process works, see [digital onboarding](https://digital.nhs.uk/developer/guides-and-documentation/digital-onboarding#using-the-digital-onboarding-portal).