shuf: move help strings to markdown file

[chenchiii]

#4368

shuf -h outputs the following.

$ ./target/debug/coreutils shuf -h
Shuffle the input by outputting a random permutation of input lines.
Each output permutation is equally likely.
With no FILE, or when FILE is -, read standard input.

Usage: ./target/debug/coreutils shuf [OPTION]... [FILE]
./target/debug/coreutils shuf -e [OPTION]... [ARG]...
./target/debug/coreutils shuf -i LO-HI [OPTION]...;

Arguments:
  [file]  

Options:
  -e, --echo [<ARG>...]       treat each ARG as an input line
  -i, --input-range <LO-HI>   treat each number LO through HI as an input line
  -n, --head-count <COUNT>    output at most COUNT lines
  -o, --output <FILE>         write result to FILE instead of standard output
      --random-source <FILE>  get random bytes from FILE
  -r, --repeat                output lines can be repeated
  -z, --zero-terminated       line delimiter is NUL, not newline
  -h, --help                  Print help information
  -V, --version               Print version information

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tail-2/inotify-dir-recreate is no longer failing!