[Feature] Update iOS Guidelines, groundwork for Setup Intent and Payment Intent

[beachmongoose]

Is your feature request related to a problem? Please describe.

• No action is required regarding the SDK itself. This is just in regards to documentation.
• We recently enabled Cash App integration via Stripe. It took longer than anticipated because the documentation around setupIntent and paymentIntent is vague.
• Issues included
  • return_url setup and handling
  • paymentIntent is presented as the main method, had to do some searching to find setupIntent. Would make more sense if they were together.
  • clientSecret retrieval
  • Using STPAuthenticationContext when bulk of logic is handled by an object that isn't a view. Passed in the related view, but making it conform to STPAuthenticationContext to call required func authenticationPresentingViewController led to error message that module 3DS2 (3D Secure 2) was missing.

Describe the solution you'd like

• Updates to the Stripe Cash App documentation to outline the preliminary steps required to allow the use of Setup Intent and Payment Intent.
• Troubleshooting guide for errors related to missing 3DS2 module.

Describe alternatives you've considered

• Attempting to ask the AI on the site led to mixed results and some misinformation.

Additional context

• We have things set up on our end, so we don't require further assistance. But I'd like things to be easier for the next group that comes around.

[cttsai-stripe]

@beachmongoose Thank you for the suggestion! We revisit our roadmap periodically and will keep this idea in mind.