Add a pre-built crate for contributors to test on

[jyn514]

This would make contributing much less painful since you don't need docker or the rustwide utils, you could just test what the crate looks like without having to run cargo run build crate first.

I think the fakes module already does most of what this needs, maybe we could combine that with the regex.html that's already in benches?

[cynecx]

I was quite surprised that cargo run -- build crate regex 1.3.9 would somehow install a rust toolchain through my locale rustup. That's quite unexpected? Because this could be offloaded to a docker container.

[jyn514]

It's not through your local rustup, it also downloads its own copy of rustup 😆 the reason is that documentation is built with the latest nightly, which you might not have installed.

It would be nice to make it part of a docker container, but then it would get out of date whenever a new nightly was released. So I'm not sure that's feasible.

[cynecx]

Ah, nevermind. I thought it touched my local rustup. I wasn't able to actually correctly run cargo run -- build crate regex 1.3.9 because my docker configuration contained:

{
    "userns-remap": "default"
}

sigh

[syphar]

With #3187 we don't need this any more