Add missing links for namespace aliases and extern alias in C# namespaces guide (#48060)

* Fixed bug 48043.

* Update docs/csharp/fundamentals/types/namespaces.md

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Co-authored-by: Adit Sheth <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>

Author: 3 people

docs/csharp/fundamentals/types/namespaces.md

@@ -18,7 +18,10 @@ Namespaces are heavily used in C# programming in two ways. First, .NET uses name
 
 :::code language="csharp" source="snippets/namespaces/Program.cs" ID="Snippet23":::
 
-For more information, see the [using Directive](../../language-reference/keywords/using-directive.md).
+For more information, see the [using directive](../../language-reference/keywords/using-directive.md).
+
+You can also create an **alias** for a namespace or type using the [using alias directive](../../language-reference/keywords/using-directive.md#the-using-alias).  
+In more advanced scenarios, you can reference multiple assemblies with the same namespaces or types by using the [extern alias](../../language-reference/keywords/extern-alias.md) feature.
 
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