The Book should have a page for Traits, `/book/traits`

[novusnota]

Like #115, but for Traits.

• Differences with contracts:
  • no init(), that's only for contracts
  • keywords: virtual and abstract (in trait) and override (in contract)
  • @interface — same as in contracts, refer to them from this page OR move description to traits.
• Links to commonly used traits from stdlib (as they've explained well in Reference → Standard Libraries)
• ~~Description of the base trait, which all contracts implicitly implement~~ → covered in Core library