Add plots for benchmarks of different machines to README

[efaulhaber]

Preview at https://trixi-framework.org/PointNeighbors.jl/previews/PR169/.
Note that the README.md doesn't show the new plots until merged into main, but they can be previewed on the starting page of the docs linked above.

This is the first PR revising the docs. In future PRs, I will

• update the other benchmark plots and generate them in CI,
• add the MI300A benchmarks,
• add TLSPH benchmarks for CPU once Add parallel intitialization for TLSPH arrays to ensure NUMA awareness TrixiParticles.jl#1256 is merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.59%. Comparing base (78be5a6) to head (4350a65).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #169   +/-   ##
=======================================
  Coverage   84.59%   84.59%           
=======================================
  Files          15       15           
  Lines         792      792           
=======================================
  Hits          670      670           
  Misses        122      122           

Flag	Coverage Δ
unit	84.59% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[svchb]

I don't see the point in merging the code for the plots. You could just add them to an issue like we do it in TrixiParticles.jl and also include the numbers and code there.

[efaulhaber]

Codex Review

The PR separates benchmark plotting from benchmark execution, adds machine-comparison plots to the documentation workflow, and embeds those generated plots in the README and docs. The split leaves the documented plotting workflow broken because its include no longer defines the plotting function.

This is an AI-generated code review. Please verify the findings and summary before acting on them.
_{Review generated by codex-pr-review}

[efaulhaber]

Codex Review

The PR separates plotting from benchmark execution, adds CI-generated cross-machine benchmark plots to the documentation, and updates README benchmark and GPU-compatibility information. No concrete correctness or regression issues were found.

This is an AI-generated code review. Please verify the findings and summary before acting on them.
_{Review generated by codex-pr-review}