Add Payment Element integration documentation

[youssef-al-mostafa]

Closes laravel/cashier-stripe#1541

The current documentation only covers the legacy Card Element which Stripe
now marks as "Legacy". This PR adds a new "Using the Payment Element" section
covering the three main use cases:

• Subscriptions (Setup Intent flow)
• Single charges (Payment Intent flow)
• Updating the default payment method

The legacy Card Element docs are kept intact for backward compatibility.

[taylorotwell]

So does this leave the old legacy stuff in the docs as well?

[youssef-al-mostafa]

Yes, I kept the old Card Element docs in place since a lot of existing apps still use it.
Added the Payment Element as a new section below it so new projects have a modern reference without breaking anything for current users.
Happy to add a small deprecation note to the old section pointing to the new one if you'd prefer that approach.

[taylorotwell]

Can you fix the styling to make it match the rest of the docs?

[youssef-al-mostafa]

Updated formatting and prose style to match the existing docs. Ready for review.