Rearrange incident management nav items

Make incident management nav items follow the structure discussed with the grouprespond PM and Product Designer in gitlab#374994 (closed)

Before	After

• Link to the merge request that introduced the new page:
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://msj-respond-nav-items.docs.gitlab-review.app/ee/operations/incident_management/