don't lookup anchors in includes that are not found (or are cyclical)

Author: Mpdreamz

src/Elastic.Markdown/IO/MarkdownFile.cs

@@ -227,6 +227,7 @@ public static List<PageTocItem> GetAnchors(
 	{
 		var includeBlocks = document.Descendants<IncludeBlock>().ToArray();
 		var includes = includeBlocks
+			.Where(i => i.Found)
 			.Select(i =>
 			{
 				var path = i.IncludePathFromSourceDirectory;