sleep: move help strings to markdown file

[chenchiii]

#4368

sleep -h outputs the following.

$ ./target/debug/coreutils sleep -h
Pause for NUMBER seconds.

Usage: ./target/debug/coreutils sleep NUMBER[SUFFIX]...
./target/debug/coreutils sleep OPTION

Arguments:
  [NUMBER]...  pause for NUMBER seconds

Options:
  -h, --help     Print help information
  -V, --version  Print version information

Pause for NUMBER seconds.  SUFFIX may be 's' for seconds (the default),
'm' for minutes, 'h' for hours or 'd' for days.  Unlike most implementations
that require NUMBER be an integer, here NUMBER may be an arbitrary floating
point number.  Given two or more arguments, pause for the amount of time
specified by the sum of their values.