Change user facing docs to use Duo Agent Platform instead of Duo Workflow

Problem

As laid out in &18255 the product name is changing from GitLab Duo Workflow to GitLab Duo Agent Platform. The most important point here are our user facing docs which are still focused on GitLab Duo Workflow and hence confusing.

Desired Outcome

https://docs.gitlab.com/user/duo_workflow/ and related pages should no longer talk about Duo Workflow.

Additionally we need to make it clear, that the current feature in the IDE is only a slice of the solution.

E.g. call it out as the Software Development flow in the IDE.