[clang-tools-extra] [clang-doc] Add Markdown AST node type definitions (PR #205609)

[Paul Kirth via cfe-commits]

================
@@ -0,0 +1,56 @@
+#include "support/Markdown.h"
+#include "llvm/Support/Casting.h"
----------------
ilovepi wrote:

I don't think you need this header.

https://github.com/llvm/llvm-project/pull/205609