Improve the Cashier (Paddle) documentation

[carlosmintfan]

• Change (probably invalid) integer prices to strings
• Explain difference between $user->onTrial() and $user->subscription()->onTrial()
• Explain default payment link and that a blank page with the paddleJS blade directive should suffice
• Update payment method without redirecting to management url

[carlosmintfan]

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) and Add examples for retrieving information about single charges applied to a subscription will folllow up in another PR.

[taylorotwell]

Hmm, I don't find the language about the blank page entirely clear.