Change development docs from Duo Workflow to Duo Agent Platform

Problem

As laid out in &18255 the product name is changing from GitLab Duo Workflow to GitLab Duo Agent Platform.

With user facing docs being changed in #551382 (closed) we'll also need to make sure to update developer facing docs to avoid confusion between the different concepts:

https://docs.gitlab.com/development/duo_workflow/

Desired Outcome

As far as possible https://docs.gitlab.com/development/duo_workflow/ and associated pages are cleaned up to use the terms established in !193744 (diffs).