88 Added documentation and updated benchmark pipeline for rotating-cylinders 2D

[Sarbani-Roy]

Closes #88

Summary

Adds documentation for the rotating-cylinders 2D benchmark and scripts to generate plots for DuMux and OpenFOAM.

Changes

Documentation (docs/benchmarks/rotating_cylinders/)

• Problem description, analytical solution, mesh, and boundary conditions
• Numerical results for DuMux and OpenFOAM: L² error tables, convergence
  plots, radial profiles, and 2D field plots
• Cross-solver convergence comparison
• References: Taylor (1923), OpenFOAM documentation, DuMux benchmark repository

Benchmark cases (benchmarks/rotating-cylinders/)

• OpenFOAM case files consolidated into output_template.zip
• compute_l2_error.py added for OpenFOAM L² error computation; outputs
  solution_metrics.json consistent with the DuMux pipeline
• plot_results.py added for both solvers to generate documentation plots
• plot_convergence.py added for per-solver and cross-solver convergence plots
• Snakefiles, run scripts, and GitHub Actions workflow updated accordingly

Notes

• plot_results.py and plot_convergence.py are used for plot generation related to documentation but can be removed if deemed redundant