[gn] port a1d3cdc

Author: nico

llvm/utils/gn/secondary/clang-tools-extra/clang-doc/tool/BUILD.gn

@@ -7,6 +7,7 @@ copy("assets") {
     "../assets/enum-template.mustache",
     "../assets/function-template.mustache",
     "../assets/head-template.mustache",
+    "../assets/index-template.mustache",
     "../assets/index.js",
     "../assets/mustache-index.js",
     "../assets/namespace-template.mustache",