WIP: Improve the Cashier (Paddle) documentation

[carlosmintfan]

TODO:

• Change (probably invalid) integer prices to strings
• Add examples for retrieving information about single charges applied to a subscription
• Warn about $user->subscription() being nullable
• Explain difference between $user->onTrial() and $user->subscription()->onTrial()
• Improve the bill for non-catalog items PR (it must also show how to add those to subscriptions and check which were added if possible)
• Explain default payment link and that a blank page with the paddleJS blade directive should suffice
• Update payment method without redirecting to management url
  Possibly more...