Docs: update recommended Clang version

[merlinND]

Hi,

@jhoydis pointed out recently that the current build guide's instructions:

sudo apt install clang-10 libc++-10-dev libc++abi-10-dev cmake ninja-build

no longer work on e.g. Ubuntu 22.04 (and most likely more recent versions as well), because the libc++-10-dev and libc++abi-10-dev packages no longer exist.

I understand that Clang 10 is used in the CI and it makes sense to recommend the same compiler version that is tested, but since it's not working anymore, would switching to Clang 15 in the docs be okay?