uname: move help strings to a markdown file

[IZUMI-Zu]

#4368
After this PR, uname -h gives the following output.

./target/debug/coreutils uname -h
Print certain system information.
With no OPTION, same as -s.

Usage: ./target/debug/coreutils uname [OPTION]...

Options:
  -a, --all               Behave as though all of the options -mnrsvo were specified.
  -s, --kernel-name       print the kernel name.
  -n, --nodename          print the nodename (the nodename may be a name that the system is known by to a communications
                          network).
  -r, --kernel-release    print the operating system release.
  -v, --kernel-version    print the operating system version.
  -m, --machine           print the machine hardware name.
  -o, --operating-system  print the operating system name.
  -h, --help              Print help information
  -V, --version           Print version information

[tertsdiepraam]

Nice! Just one small suggestion

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/tee. tests/misc/tee is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/tee. tests/misc/tee is passing on 'main'. Maybe you have to rebase?