Update docs for the Luhn exercise

[tasxatzial]

Forum topic https://forum.exercism.org/t/luhn-fix-instructions/16242

Summary:

• Updated the instructions: "credit card numbers" was changed to "numbers"
• Removed references to fraud from the introduction. According to Wikipedia, the algorithm was designed to protect against accidental errors, not malicious attacks.
• Updated the headings: There is now a level 2 main heading called "Examples," with each example starting with a level 3 heading.
• The original examples both used valid/invalid credit card numbers. Since they featured the same type of numbers with an even number of digits, the second example was replaced with an invalid Canadian SIN that has nine digits to better demonstrate the algorithm.
• Refined the wording throughout the instructions to improve clarity.