[Docs] Update e-commerce tutorial to use new Stripe Checkout

[thorsten-stripe]

Description

@jlengstorf as discussed I'm submitting a PR to update the Gatsby e-commerce tutorial in advance of our live stream on Jan 14.

Changes made

• Add easy one button example
• Add advanced example that uses the gatsby-source-stripe plugin
• Removed example that requires a cloud function but kept a reference via link at the bottom

TODO

• Update https://gatsby-ecommerce.netlify.com/ with https://github.com/thorsten-stripe/ecommerce-gatsby-tutorial

[gatsbot]

Holy buckets, @thorsten-stripe — we just merged your PR to Gatsby! 💪💜

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

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Thanks again!

[shannonbux]

Hi @thorsten-stripe! Thanks for these updates! I hope I didn't jump the gun and merge this before you were finished; let me know if I did!

[thorsten-stripe]

@shannonbux thank you! Do you happen to know who owns https://gatsby-ecommerce.netlify.com/? Would be great if we could update that with the new examples: https://github.com/thorsten-stripe/ecommerce-gatsby-tutorial