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

Adds e2e tests for clang-doc, which serves as useful test against
regression in the html output. It also serves as a helpful tool for code
review as we can

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

Adds e2e tests for clang-doc, which serves as useful test against
regression in the html output. It also serves as a helpful tool for code
review as we can see the diffs to clang-docs output

Closes https://github.com/llvm/llvm-project/issues/93925

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