BUG: Fix argument handling in nxrunner

[imikejackson]

Summary

Consolidates the two nearly-identical SIMPL pipeline conversion options in nxrunner into a single --convert / -c flag and cleans up the related help output.

What changed

• Merged --convert-output / -co into --convert / -c. Previously there were two flags whose only difference was print-to-stdout vs. write-to-file. The confusingly similar pair (--convert vs --convert-output, -c vs -co) was an argument-handling hazard. --convert now does the useful thing: it writes the converted pipeline to a .d3dpipeline file next to the input.
• Removed the now-dead ConvertOutput argument type, its parsing branch, its help-menu dispatch, the orphaned DisplayConvertOutputHelp() function, and the unused k_ConvertOutputParam{Long,Short} constants.
• Fixed stale user-facing messages in the execute/preflight paths that told users to run the removed --convert-output flag; they now reference --convert.
• Fixed help-text alignment by replacing the ragged leading \t (tab + space) prefixes with consistent two-space indentation.
• Updated docs/NX_CLI.md to document the single --convert flag and its save-to-file behavior.

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