[Refactor] Extract GetDisplayAvatarUrl in IUser (#2771)

* Implement `GetDisplayAvatarUrl` method

* Remove obsolete example

* Update XML documentation

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Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>

src/Discord.Net.Rest/Entities/Users/RestUser.cs

@@ -142,6 +142,9 @@ namespace Discord.Rest
                 ? CDN.GetDefaultUserAvatarUrl(DiscriminatorValue)
                 : CDN.GetDefaultUserAvatarUrl(Id);
 
+        public virtual string GetDisplayAvatarUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
+            => GetAvatarUrl(format, size) ?? GetDefaultAvatarUrl(); 
+            
         /// <inheritdoc />
         public string GetAvatarDecorationUrl()
             => AvatarDecorationHash is not null