Revise troubleshooting section

[christopher-hakkaart]

This pull request improves the documentation in docs/cache-and-resume.md by restructuring the section on cache failures, providing clearer explanations, and adding examples to help users troubleshoot common issues. The most important changes include reorganizing the causes of cache failures into subsections, enhancing clarity with examples, and offering solutions for resolving specific failure types.

Improvements to documentation structure and clarity:

• Reorganized the "Cache Failures" section by grouping causes into subsections, such as "Resume not enabled," "Non-default cache directives," and "Modified input files," for better readability.
• Added detailed explanations and examples for each failure type, such as how to enable -resume, handle non-default cache directives, and avoid modifying input files.

Examples and solutions for specific failure types:

• Provided examples and solutions for handling race conditions caused by global variables, including using local variables or avoiding variables entirely.
• Clarified the behavior of non-deterministic process inputs and added a solution to join channels explicitly before invoking a process.

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[bentsherman]

@pditommaso please do not merge significant docs PRs without allowing me to review. I had to make several corrections in #6299