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

[Paul Kirth via cfe-commits]

ilovepi wrote:

Petr and I spoke offline. https://github.com/llvm/llvm-project/pull/95187, is still independently useful, since it matches our normal conventions about the build directory matching the install directory. 

I think we agree on it not being a fatal error, though if it can generate HTML without those assets. https://github.com/llvm/llvm-project/pull/94717, is also useful.

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