[clang] [clang/www/get_started.html] Use newer `cmake` syntax (PR #93503)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll requested changes to this pull request.

I'm fine with `cmake --build`, but I don't think that replacing `cd` with `-S` and `-B` is an improvement.

https://github.com/llvm/llvm-project/pull/93503