fix: Always use UTF-8 encoding when saving documents

Use Encoding.UTF8 instead of Encoding.Default as the latter one
might return a non-Unicode encoding on .NET Framewrok causing
encoding errors when saving non-ASCII characters to a document.

Author: ap0llo

src/MarkdownGenerator/_Model/MdDocument.cs

@@ -105,7 +105,7 @@ public void Save(string path, MdSerializationOptions serializationOptions)
         /// <param name="serializationOptions">The options to use for serialization.</param>
         public void Save(Stream stream, MdSerializationOptions serializationOptions)
         {
-            using (var writer = new StreamWriter(stream, Encoding.Default, 1024, true))
+            using (var writer = new StreamWriter(stream, Encoding.UTF8, 1024, true))
             {
                 Save(writer, serializationOptions);
             }