Add fmt.Scanf input example

[IntegerAlex]

add a new Input from Program example that demonstrates scanning interactive user input with fmt.Scanf, including basic validation and error handling

[IntegerAlex]

Fix - Feat: #637

[eliben]

Thanks, but this is something I have to think about.

Generally the idea of gobyexample isn't to give examples for every possible stdlib function, and fmt already has pretty good examples (e.g. see https://pkg.go.dev/fmt#example-Fscanf)

Personally I find that the *scan* functions aren't commonly used in Go, and bufio.Scanner is much more common (and we already have examples of that)

[eliben]

Why "Input from Program"? What program is this input from?