Document or automate the update process for WASM binaries

[nbacquey]

When writing #494, I once again found myself unable to update the WASM grammar for OCaml, despite the instructions in CONTRIBUTING.md. I struggled with Docker configurations, manual ad hoc copy/pastes, and achieved nothing in the end.

I think we need to either automate the process, or make it fool-proof.

[Xophmeister]

I struggled with Docker configurations

I think the process launches a container, rather than you having to run everything inside a container.

That said, I agree: it took a while to work out, the first time I did it. Having it part of the CI build would be ideal, if it can be triggered only when the dependency changes.

[ErinvanderVeen]

Resolved with #610