Before this change the help for convmv was generated by running the
examples each time rclone started up. Unfortunately this involved
running the echo command which did not work on Windows.
This pre-generates the help into `transform.md` and embeds it. It can
be re-generated with `go generate` which is a better solution.
See: https://forum.rclone.org/t/invoke-of-1-70-0-complains-of-echo-not-found/51618
