Update Duo Self-Hosted routing for unified Duo page

Description

Implement the necessary routing changes to support the UI for unified Duo settings (option 2).

Currently Duo self-hosted features live under admin/ai/duo_self_hosted. As a part of the unification process, these pages will need to be relocated to nest under the root admin/gitlab_duo page.

See the seat utilization or Duo configuration page as an example (admin/gitlab_duo/seat_utilization, admin/gitlab_duo/configuration).

Proposal

• The main GitLab Duo Self-Hosted page would relocate to admin/gitlab_duo/self_hosted. The AI feature settings tab will be the default open tab on this page so will not have its own route.
• Self-hosted models tab will relocate to admin/gitlab_duo/self_hosted/models
• Self-hosted models create page will relocate to admin/gitlab_duo/self_hosted/models/new
• Self-hosted models edit page will relocate to admin/gitlab_duo/self_hosted/models/:modelId/edit

Definition of Done:

• Routing for the Duo self-hosted is nested under the root admin/gitlab_duo pages, as per the proposal.
• UI breadcrumbs match URL routing
• Documentation updates (if necessary)