[llvm] [Github] Upload built docs as artifact from test build docs job (PR #118159)

[Louis Dionne via llvm-commits]

ldionne wrote:

This broke the documentation build for libc and for libc++. To test this, dummy documentation jobs should have been triggered by touching stuff in e.g. `libcxx/docs`. Just a heads up: I'll try fixing this for libc++, but it's possible that other people end up here after some head scratching!

https://github.com/llvm/llvm-project/pull/118159