Improve Stan conversion and docs

[sethaxen]

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Currently we convert Stan's internal statistics names to the ones used by arviz.from_cmdstan. However, those names don't follow ArviZ's schema and will eventually be updated on the ArviZ side. We should update them for Stan.jl and CmdStan.jl.

We should also switch the tests and docs from CmdStan.jl to Stan.jl, since that's the future of Julia's Stan integration.

[sethaxen]

Relates https://github.com/arviz-devs/arviz/pull/1063

[sethaxen]

This is now irrelevant, since conversion from stan types is provided by StanSample itself. See https://github.com/StanJulia/StanExampleNotebooks.jl/blob/main/notebooks/DimensionalData/dimensionaldata.jl for an example notebook.