nl: move help strings to a markdown file

[wuseyu]

#4368

After this PR, nl --help gives the following output.

Number lines of files

Usage: ./target/debug/coreutils nl [OPTION]... [FILE]...

Options:
      --help                           Print help information.
  -b, --body-numbering <SYNTAX>        use STYLE for numbering body lines
  -d, --section-delimiter <CC>         use CC for separating logical pages
  -f, --footer-numbering <STYLE>       use STYLE for numbering footer lines
  -h, --header-numbering <STYLE>       use STYLE for numbering header lines
  -i, --line-increment <NUMBER>        line number increment at each line
  -l, --join-blank-lines <NUMBER>      group of NUMBER empty lines counted as one
  -n, --number-format <FORMAT>         insert line numbers according to FORMAT
  -p, --no-renumber <no-renumber>      do not reset line numbers at logical pages
  -s, --number-separator <STRING>      add STRING after (possible) line number
  -v, --starting-line-number <NUMBER>  first line number on each logical page
  -w, --number-width <NUMBER>          use NUMBER columns for line numbers
  -V, --version                        Print version information

[sylvestre]

please remove the useless space at the end