csplit: move help strings to markdown file

[papparapa]

#4368

csplit -h outputs the following.

$ ./target/debug/coreutils csplit -h
Split a file into sections determined by context lines

Usage: ./target/debug/coreutils csplit [OPTION]... FILE PATTERN...

Options:
  -b, --suffix-format <FORMAT>  use sprintf FORMAT instead of %02d
  -f, --prefix <PREFIX>         use PREFIX instead of 'xx'
  -k, --keep-files              do not remove output files on errors
      --suppress-matched        suppress the lines matching PATTERN
  -n, --digits <DIGITS>         use specified number of digits instead of 2
  -s, --quiet                   do not print counts of output file sizes [aliases: silent]
  -z, --elide-empty-files       remove empty output files
  -h, --help                    Print help information
  -V, --version                 Print version information

Output pieces of FILE separated by PATTERN(s) to files 'xx00', 'xx01', ..., and output byte counts of each piece to standard output.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/misc/timeout is no longer failing!