Descent 3 Build

Copy command-line documentation into `USAGE.md` & add documentation for `-additionaldir` #2002

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Run time

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Job	Run time
Windows-x64, Debug	6m 58s
Windows-x64, Release	8m 8s
macOS-Intel, Debug	11m 57s
macOS-Intel, Release	11m 13s
macOS-ARM, Debug	2m 10s
macOS-ARM, Release	2m 7s
Linux-x64, Debug	2m 50s
Linux-x64, Release	3m 30s
Linux-x64-clang, Debug	3m 28s
Linux-x64-clang, Release	3m 39s
Linux-cross-arm64, Debug	3m 27s
Linux-cross-arm64, Release	4m 37s
	1h 4m 4s