Don't build JLL library in Documenter CI workflow

[glennmoy]

In #194 we replaced the Artifacts.toml with a dummy file as main is expected to be used with Overrides.toml.

As a consequence, that Documenter.yml now has to build the ray_julia JLL for it to build the docs., which we've mitigated by caching the build artifacts similar to how we do in the regular CI.

In future we should either pull the artifacts from somewhere else: either a build cache, release assets, or artifacts from the CI job.