Documentation: Move helloworld.cx example to before REPL tutorial

[SkycoinSynth]

"Running CX programs" section must be before REPL tutorial section

• a small "heloworld" program they can copy and paste and run should be included
• the hello world program should also be in the examples folder

Section about REPL should be renamed REPL tutorial

Example of "cx help" or "cx --help" to list commands, must be given before "running CX program"

Example of "cx version" to print version should be given, after example of "cx help" to get command line arguments

[arfan499]

Done