Add wasm32-unknown-unknown target on setup

[goosethedev]

The Tooling > Checklist section mentions ensuring the installation of the wasm32-unknown-unknown "Rust toolchain".

1. The Getting Started page does not indicate how to set it up.
2. It is a compilation target, not a toolchain.

[jkelleyrtp]

Thanks for the PR!

I think this might be better served as its own section. Maybe "install toolchains" which covers all the targets.

That way you know what to install for iOS, android, and web since those are each cross-compile targets.

[goosethedev]

Maybe there should be a single Introduction > Prerequisites setup page with all tooling information (IDE, rustup, toolchains, targets, dioxus-cli, etc)? Currently that info is under the Getting Started page.

Also, that way we could get rid of the Tutorial > Tooling setup page, since it would become redundant.

Let me know what you think. I can re-structure the pages with the current info in them, but I don't think I can detail much about other compilation targets, I'm still delving into Dioxus and Rust in general.