[9.x] add guest checkout docs

[alexanderkroneis]

This PR adds a low level documentation of guest checkout which will be released in a few in laravel/cashier-stripe.

[taylorotwell]

I find these docs a bit sparse. What are $items? Can we show an actual example? What might the application do after a guest completes the checkout?

[alexanderkroneis]

I‘ll provide some further details, thank you for your improving commit. 👏

[alexanderkroneis]

I improved the documentation a little more, still not perfect imho.

[driesvints]

@alexgaal can you remove the .idea folder?

See https://sebastiandedeyne.com/setting-up-a-global-gitignore-file/

[alexanderkroneis]

@alexgaal can you remove the .idea folder?

See https://sebastiandedeyne.com/setting-up-a-global-gitignore-file/

Thank you for that tip, I just added .idea to my global gitignore. 🎉

[driesvints]

I tried to improve this even further by referencing the checkout.session.completed at the end and how to fulfill orders. @taylorotwell I don't think we need to explain $items as we don't do that anywhere in the other checkout docs. It should be pretty obvious that you can pass prices.