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

[Paul Kirth via cfe-commits]

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@@ -77,18 +102,28 @@ class AnonClass {};
 // HTML-ANON-INDEX-NOT: <h2 id="Functions">Functions</h2>
 // HTML-ANON-INDEX-NOT: <h3 id="{{([0-9A-F]{40})}}">anonFunction</h3>
 // HTML-ANON-INDEX-NOT: <p>void anonFunction()</p>
+// MD-MUSTACHE-ANON-INDEX: # namespace @nonymous_namespace
+// MD-MUSTACHE-ANON-INDEX:  Anonymous Namespace
+// MD-MUSTACHE-ANON-INDEX: ## Records
+// MD-MUSTACHE-ANON-INDEX: * [AnonClass](AnonClass.md)
+// MD-MUSTACHE-ANON-INDEX: ## Functions
+// MD-MUSTACHE-ANON-INDEX: ### anonFunction
+// MD-MUSTACHE-ANON-INDEX: *void anonFunction()*
+
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ilovepi wrote:

```suggestion
```
extra newline

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