readlink: move help strings to markdown file

[m11o]

issue: #4368

This PR contains bellow contents.

• Move help strings to markdown file e82ba6e
• Use override_usage method instead of override_help method b84a94b

readlink --help outputs follow.

before

$ ./target/debug/readlink --help
./target/debug/readlink [OPTION]... [FILE]...

after

$ ./target/debugreadlink --help
Print value of a symbolic link or canonical file name.

Usage: ./target/debug/readlink [OPTION]... [FILE]...

Arguments:
  [files]...

Options:
  -f, --canonicalize           canonicalize by following every symlink in every component of the given name recursively; all but the last component must exist
  -e, --canonicalize-existing  canonicalize by following every symlink in every component of the given name recursively, all components must exist
  -m, --canonicalize-missing   canonicalize by following every symlink in every component of the given name recursively, without requirements on components existence
  -n, --no-newline             do not output the trailing delimiter
  -q, --quiet                  suppress most error messages
  -s, --silent                 suppress most error messages
  -v, --verbose                report error message
  -z, --zero                   separate output with NUL rather than newline
  -h, --help                   Print help
  -V, --version                Print version

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail-2/inotify-dir-recreate. tests/tail-2/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?

[tertsdiepraam]

Excellent! Thanks!