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

[Paul Kirth via cfe-commits]

https://github.com/ilovepi commented:

Also, I'm not sure I understand the need for empty files. If we don't need them, then lets remove them. If you need to use an empty file for some test, you can create those w/ `touch` in the test itself, but I don't see those referenced at all in `basic-project.test`.

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