[ic-side-navigation] sideNavExpanded event should use @Event so it appears in docs

[GCHQ-Developer-530]

Summary of the bug

At the moment, the sideNavExpanded event is added like this:

const event = new CustomEvent("sideNavExpanded", {
  detail: {
    sideNavExpanded: objDetails.sideNavExpanded,
    sideNavMobile: objDetails.sideNavMobile,
  },
});
this.el.dispatchEvent(event);

But this means it doesn't show up in the docs on our website, and it also isn't consistent with other components.

We should look into rewriting it so that we have @Event() sideNavExpanded and emit the event like we do elsewhere:

this.sideNavExpanded.emit(
  {
    sideNavExpanded: objDetails.sideNavExpanded,
    sideNavMobile: objDetails.sideNavMobile,
  }
)

If a rewrite like this isn't possible, we should look into how to manually get the event in its current state into docs.

The only other component that creates an event like this is top nav for topNavResized. This should be updated too.

🧐 Expected behaviour

sideNavExpanded event should show up in docs.