bpo-28626: rearrange discussion of output formatting to encourage f-strings (#6036)

The 'output formatting' section of the tutorial talks a lot about manual formatting with things like .rjust() and .zfill(), with only a passing reference to 3.6's new f-strings.

This doesn't drop all of the old material, but it does rearrange the topics into a more modern order: f-strings first, discussing formatting specifiers a bit; then calling .format(); finally manual formatting with .ljust().