Parallelization doc options simplification

[joshqsumner]

Describe your changes
Moving the cores option in config.cluster_config to be hidden, restructuring docs so that run_parallel links to helper functions and they are not listed in the main sidebar, and adding some more context to the "why parallelize" docs about how to set up for parallelization and to hit some other buzzwords like "multiprocess" and "batch process".

Type of update
This is mostly a docs/readability change.

Associated issues
None.

For the reviewer
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• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 8361dc3..2f4e58f. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	✅ Success		View Check ↗

Code Coverage Report

Metric	Aggregate	Python
Branch Coverage	100%	100%
Composite Coverage	100%	100%
Line Coverage	100%	100%
New Branch Coverage	100%	100%
New Composite Coverage	100%	100%
New Line Coverage	100%, ✅ Above Threshold	100%, ✅ Above Threshold

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