These combined LLVM+Rust toolchains are now available, thanks to Nathan
Chancellor (ClangBuiltLinux).

Thus introduce them in the Rust Quick Start guide.
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20240517170615.377786-1-ojeda@kernel.orgCo-developed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>