[JOSS] docs: Improve batch_run documentation for parallel processing

[EwoutH]

This PR enhances the documentation for Mesa's batch_run function by:

1. Adding explicit mention of the number_processes parameter in the getting started guide
2. Including an explanation of why parallel processing should be avoided in Jupyter notebooks
3. Adding a more detailed explanation in the batchrunner module docstring about multiprocessing limitations

Part of #2666, based on feedback from @martibosch.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -1.4% [-2.3%, -0.5%]	🔵 +0.1% [-0.0%, +0.3%]
BoltzmannWealth	large	🔵 -0.5% [-1.1%, -0.0%]	🔵 -0.1% [-0.5%, +0.3%]
Schelling	small	🔵 -0.4% [-0.5%, -0.2%]	🔵 +0.1% [-0.0%, +0.2%]
Schelling	large	🔵 -0.6% [-0.8%, -0.3%]	🔵 -0.5% [-1.8%, +0.8%]
WolfSheep	small	🔵 -0.4% [-0.6%, -0.2%]	🔵 -0.8% [-1.0%, -0.7%]
WolfSheep	large	🔵 -0.5% [-0.7%, -0.2%]	🔵 -0.9% [-1.4%, -0.5%]
BoidFlockers	small	🔵 -1.0% [-1.8%, -0.1%]	🔵 -0.7% [-0.9%, -0.5%]
BoidFlockers	large	🔵 -0.6% [-1.1%, -0.1%]	🔵 -0.6% [-0.8%, -0.4%]