Update benchmarking docs

[ranocha]

I made some changes to the benchmarking setups (which were bit-rotten...). This fix should document the necessary steps to benchmark a PR. Please let me know if this works for you, @DanielDoehring

[github-actions]

Review checklist

This checklist is meant to assist creators of PRs (to let them know what reviewers will typically look for) and reviewers (to guide them in a structured review process). Items do not need to be checked explicitly for a PR to be eligible for merging.

Purpose and scope

• The PR has a single goal that is clear from the PR title and/or description.
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Code quality

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Documentation

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Testing

• The PR passes all tests.
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Performance

• There are no type instabilities or memory allocations in performance-critical parts.
• If the PR intent is to improve performance, before/after time measurements are posted in the PR.

Verification

• The correctness of the code was verified using appropriate tests.
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  are posted in the PR.

Created with ❤️ by the Trixi.jl community.

[DanielDoehring]

Just submitted a run on julia 1.9.4 , let's see if this runs smoothly.

[DanielDoehring]

Thanks for the update, this works now!

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I added this information on RAM since one of my first runs crashed due to too little RAM allowed for each thread.
I think this is valuable information for anyone using this benchmarking in conjunction with a workload manager such as SLURM.

[ranocha]

Great, thanks!