Delete existing docs when generating new docs (#4022)

This commit updates the doc generation process to delete existing documentation,
except for the breaking changes directories, when generating new  documentation.

This is to prevent stale doc pages from carrying over.

Closes #3976

(cherry picked from commit 0c674cd)

Author: russcam

src/CodeGeneration/DocGenerator/LitUp.cs

@@ -68,7 +68,10 @@ public static async Task GoAsync(string[] args)
 			var projects = workspace.CurrentSolution.Projects
 				.ToDictionary(p => p.Name, StringComparer.OrdinalIgnoreCase);
 
-			foreach (var file in GetDocumentFiles(projects).SelectMany(s => s)) await file.SaveToDocumentationFolderAsync();
+			DeleteExistingDocs();
+
+			foreach (var file in GetDocumentFiles(projects).SelectMany(s => s))
+				await file.SaveToDocumentationFolderAsync();
 
 			Console.ForegroundColor = ConsoleColor.Green;
 			Console.WriteLine("Documentation generated.");
@@ -81,6 +84,20 @@ public static async Task GoAsync(string[] args)
 			}
 		}
 
+		private static void DeleteExistingDocs()
+		{
+			var outputDir = new DirectoryInfo(Program.OutputDirPath);
+
+			foreach (var file in outputDir.EnumerateFiles())
+				file.Delete();
+
+			foreach (var dir in outputDir.EnumerateDirectories())
+			{
+				if (!dir.Name.EndsWith("breaking-changes"))
+					dir.Delete(true);
+			}
+		}
+
 		private static void AddDocumentsToWorkspace(AdhocWorkspace workspace)
 		{
 			// we only need source for the Tests project.