Refine kernel tuning documentation

[nduytg]

Summary

• rename the kernel tuning markdown files to use an underscore naming pattern
• expand the kernel tuning overview with additional persistence guidance and references to kernel ceilings
• deepen the process and file descriptor limit guide with detailed inspections of soft, hard, and kernel limits
• restructure the kernel tuning guide to emphasize inspection workflows, persistence best practices, IO scheduler selection, and swap management
• replace the legacy file descriptor note with a dedicated process-and-file limits reference and remove the obsolete plain text file

Testing

• ⚠️ pre-commit run --all-files (command not found in container)

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https://chatgpt.com/codex/tasks/task_e_68ef8e35203083339da9a266f9a2f3f6