Reapply [clang-doc] Add --asset option to clang-doc (#96358)

Reapply https://github.com/llvm/llvm-project/pull/94717
Adds a new option --asset which allows users to specified the asset
folder for

Reapply [clang-doc] Add --asset option to clang-doc (#96358)

Reapply https://github.com/llvm/llvm-project/pull/94717
Adds a new option --asset which allows users to specified the asset
folder for the html output of clang-doc.

This patch adds a better test for --asset option + fixes bug where
clang-doc assumes that user supplied js file is assume to be index.js

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