Add guide for HTTP P3 #378

With the coming stabilization of wasm32-wasip3 in Rust (and the general usability of P3), it is probably useful to have a page that describes how to set get set up with P3 in a HTTP context.

Ideally we would have a P3 page that looks a bit like the P2 page (putting the whole page in a P2/P3 tab is probably excessive);
https://component-model.bytecodealliance.org/language-support/using-http-in-components/rust.html

It wasn't clear to a user that was recently trying to build a component how to build a P3 component (or even how to pull down WIT, using wit-bindgen, etc), and while this has been a somewhat moving target in the past, we should have a page that makes this very obvious.