timeout: move help strings to markdown file

[m11o]

issue: #4368

timeout --help outputs follow.

$ ./target/debug/timeout --help
Start COMMAND, and kill it if still running after DURATION.

Usage: ./target/debug/timeout [OPTION] DURATION COMMAND...

Arguments:
  <duration>
  <command>...

Options:
      --foreground               when not running timeout directly from a shell prompt, allow COMMAND to read from the TTY and get TTY signals; in this mode, children of COMMAND will not be timed out
  -k, --kill-after <kill-after>  also send a KILL signal if COMMAND is still running this long after the initial signal was sent
      --preserve-status          exit with the same status as COMMAND, even when the command times out
  -s, --signal <SIGNAL>          specify the signal to be sent on timeout; SIGNAL may be a name like 'HUP' or a number; see 'kill -l' for a list of signals
  -v, --verbose                  diagnose to stderr any signal sent upon timeout
  -h, --help                     Print help
  -V, --version                  Print version