Fix MAUI RoutedViewHost Navigation (#3692)

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**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->
Bug fix

**What is the current behavior?**
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#3640 When 'RoutedViewHost' is activated, it calls
'SyncNavigationStacksAsync', which pushes a page for the current
ViewModel. However, the Router.Navigate subscription is also triggered,
creating and pushing a new page for the same ViewModel.

**What is the new behavior?**
<!-- If this is a feature change -->
To fix this, I filtered Navigate to execute the action only when the
NavigationStacks have different sizes, since it seems that is what the
Navigate subscription was supposed to do anyway. I think this makes the
subsequent call to SyncNavigationStacksAsync() useless. Should I remove
it?

**What might this PR break?**
RoutedViewHost Navigate subscription.


**Please check if the PR fulfills these requirements**
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:
I was informed that there were no tests for MAUI and no documentation
seemed to be present either.

src/ReactiveUI.Maui/RoutedViewHost.cs

@@ -76,6 +76,7 @@ public RoutedViewHost()
 
             Router?
                 .Navigate
+                .Where(_ => Navigation.NavigationStack.Count != Router.NavigationStack.Count)
                 .ObserveOn(RxApp.MainThreadScheduler)
                 .SelectMany(_ => PagesForViewModel(Router.GetCurrentViewModel()))
                 .SelectMany(async page =>