Add examples of Default and when to/not use it

[adamchalmers]

Default is mentioned occasionally in the book, but I think it could use its own page.

[Dipanshusinghh]

Hi, I’d like to work on this issue.@adamchalmers

I think adding a separate section for the Default trait with a few simple examples would really help beginners. I’m planning to include:

• a short explanation of what Default does
• examples for basic types and a custom struct
• some notes on when it’s useful and when it might not be the best choice

Let me know if this sounds good, or if there’s anything specific you’d like me to include.