[clang-tools-extra] [clang-doc] Add a top-level index.html page (PR #171899)

[Paul Kirth via cfe-commits]

================
@@ -685,6 +734,9 @@ Error JSONGenerator::generateDocumentation(
         return Err;
   }
 
+  if (auto Err = serializeIndex(CDCtx, RootDir))
+    return Err;
+
   return Error::success();
----------------
ilovepi wrote:

```suggestion
   return serializeIndex(CDCtx, RootDir));
```
This is the same, right? I believe `serializeIndex()` returns an `Error`, so we don't need to check it here and can just return it directly.

https://github.com/llvm/llvm-project/pull/171899