[clang-tools-extra] [clang-doc] Add a Mustache Markdown generator (PR #177221)

[Paul Kirth via cfe-commits]

================
@@ -69,6 +69,8 @@ enum class CommentKind {
   CK_Unknown
 };
 
+enum OutputFormatTy { md, yaml, html, json, md_mustache };
+
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ilovepi wrote:

There's an argument that it could be an enum scoped to the ClangDocContext. I'll leave the decision up to you, since its mostly a question of style.

https://github.com/llvm/llvm-project/pull/177221