Clarify that `rust` should be installed via `rustup`, not homebrew

[bfops]

First we should confirm that homebrew rust definitely doesn't work, but assuming that it doesn't, we should add add warning or two into our docs for mac users.

[gefjon]

It is technically possible to install the wasm32-unknown-unknown target manually without Rustup, as described here. It just sucks and there's no reason to do it. Some package managers also package the target (e.g. I'm installing it with fenix). Homebrew seems to not package any non-default targets, though. And we probably don't want to be providing bespoke support for all of Apt, Homebrew, yum, Portage, the three different ways Nix packages Rust, and god knows how many others.