UI Link: Differentiate in the sidebar that this is an external link

### Description

I've added a `ui-ink` to the sidebar, and it's not clear that it'll behave differently to the `ui-page` elements in the page navigation.

Whilst not all users will want this, we should at least provide an option to show an "external-link" icon or something to the right of the text to signify this will navigate away from the Dashboard.

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

Start by locating the sidebar rendering for ui-link elements and compare it with the ui-page navigation shown in the issue screenshot. Determine how an optional external-link indicator can be exposed and rendered, then verify that sidebar links visibly distinguish navigation away from the Dashboard without changing the default behavior.

Written by the indexing model from the issue text.