[clang-tools-extra] [clang-doc] Add basic e2e test (PR #93928)

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PeterChou1 wrote:

I think this finally works i think we could also shelve https://github.com/llvm/llvm-project/pull/94717
since I wrote that pr to get around the fact that we can't copy assets



https://github.com/llvm/llvm-project/pull/93928