Add example LocalPreferences.toml file to parallelization.md

[jlchan]

The example LocalPreferences.toml file is for the Rocinante workstation, and was created during a previous Trixi hackathon.

[github-actions]

Review checklist

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Code quality

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Testing

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Performance

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Verification

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  are posted in the PR.

Created with ❤️ by the Trixi.jl community.

[sloede]

Thanks!