[llvm] docs/GettingStarted: document linker-related cmake options (PR #80932)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I agree that `LLVM_PARALLEL_LINK_JOBS` is a frequently used option, but the option documentation duplication makes me nervous (status quo). I wonder whether this section should just list option names and refer readers to `CMake.html`.

https://github.com/llvm/llvm-project/pull/80932