feat: integrate with alert-api

[pjungermann]

Integrating the announcements with the alert-api by Backstage (core-plugin-api) would allow to display them at any view. This would not require a "home" page with a provided announcement widget to be used.

[chipnesh]

Agree.
For now, in order to display it as shown in the documentation example, I need to place the NewAnnouncementBanner on every single page.

Alternatively, I can place it inside the Root component, but this will alter the page layout and cause it to appear above the header.

<AppRouter>
    <VisitListener/>
    <Root>
        <NewAnnouncementBanner category='public' max={3}/>
        {routes}
    </Root>
</AppRouter>