Add DetailLayout component to alerts detail page

What does this MR do and why?

Background

!231646 (closed) explored adding DetailLayout Vue and ::Layouts::DetailLayout View components. It identified:

1. Layouts should be responsible for basic loading state and supporting <template #loading> slot for individual implementations to render <gl-skeleton-loader> as they set fit. The loading prop ensures consistent use of <gl-loading-icon> size and margin.
2. CSS styles duplicated between layout components could diverge over time and require manual work to keep in-sync. Using layout classes (like component classes in GitLab UI) allows us to keep styles consistent between Vue and View implementations.
3. A #before slot is required to render items in layouts before <page-heading>.
4. Some views require more complex implementations of a page heading. Wrapping <h1> in page heading component with <slot name="heading-wrapper"> allows complex implementations to override the <h1> with multiple element etc. This occurs in work item detail which renders multiple components.

Changes

• Add DetailLayout component to alerts detail page

Iterations

1. !233013 (merged) Add loading prop to IndexLayout components/Move styles to layout stylesheets
2. !233015 (merged) Detail layout Vue/View components
3. !233016 Add detail layout to alerts detail page 👈 you are here
4. !233018 (Draft) Add detail layout to work item detail page (coordinate with plan)
5. Base layout component??? (stretch goal) #597890

References

• gitlab-org#19509
• gitlab-org#19990
• gitlab-org/gitlab-services/design.gitlab.com#3187
• #596902 (closed)
• #596903 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Go to Alerts management http://gdk.test:3000/gitlab-org/gitlab-shell/-/alert_management/
• Create an alert https://docs.gitlab.com/operations/incident_management/integrations/#configuration
• View alerts detail page http://gdk.test:3000/gitlab-org/gitlab-shell/-/alert_management/1/details/overview

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.