feat: Use PlantUML Online Server as default configs (#10395)

chore: set plantuml config default values

Author: filzrev

src/Docfx.MarkdigEngine.Extensions/PlantUml/PlantUmlExtension.cs

@@ -18,7 +18,7 @@ public class PlantUmlOptions
 
     [JsonProperty("remoteUrl")]
     [JsonPropertyName("remoteUrl")]
-    public string RemoteUrl { get; set; }
+    public string RemoteUrl { get; set; } = "http://www.plantuml.com/plantuml/";
 
     [JsonProperty("localPlantUmlPath")]
     [JsonPropertyName("localPlantUmlPath")]
@@ -30,7 +30,7 @@ public class PlantUmlOptions
 
     [JsonProperty("renderingMode")]
     [JsonPropertyName("renderingMode")]
-    public RenderingMode RenderingMode { get; set; }
+    public RenderingMode RenderingMode { get; set; } = RenderingMode.Remote;
 
     [JsonProperty("delimitor")]
     [JsonPropertyName("delimitor")]