Add a bunch of small examples showing typical use cases

[irevoire]

• Integration with actix_web
• Show how to implements the Deserr trait manually
• Validating a struct with multiple fields that shares a relation
• Ensure a struct can't be deserialised under certain conditions (like our settings with the Checked / Unchecked thingy)
• Show a usage of the from attribute to ensure a field is never constructed if it doesn't follow certains rules (a String that can't contains certain characters like our IndexUid?)