Add documentation for nomad-pack template functions #518

Job	Run time
get-product-version	4s
Determine Go toolchain version	5s
generate-metadata-file	25s
generate-ldflags	3s
Go 1.21.5 freebsd amd64 build	52s
Go 1.21.5 linux arm build	1m 28s
Go 1.21.5 freebsd arm build	30s
Go 1.21.5 darwin amd64 build	1m 13s
Go 1.21.5 linux arm64 build	1m 34s
Go 1.21.5 freebsd arm64 build	58s
Go 1.21.5 linux amd64 build	1m 34s
Go 1.21.5 windows amd64 build	55s
Go 1.21.5 darwin arm64 build	2m 20s
Go 1.21.5 windows arm64 build	48s
Docker arm default release build	51s
Docker arm64 default release build	46s
Docker amd64 default release build	36s
	15m 2s

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