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

[Justin Bogner via cfe-commits]

bogner wrote:

This test fails on multi-config cmake generators (`cmake -G 'Visual Studio` etc), exposing an existing bug in clang-doc. Since binaries end up under a subdirectory like `build/Release/bin` but the default json and such end up in `build/share/clang-doc`, running `clang-doc` from the bin dir doesn't find the default configs.

There's one attempt to fix this here: https://github.com/llvm/llvm-project/pull/97505

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