feat: Document Combine support for iOS

[palpatim]

NOTE: THIS MUST NOT BE MERGED UNTIL iOS 1.1.0 IS RELEASED

This PR adds documentation for the soon-to-be-released Combine support in iOS.

It adds two major structural changes:

• It adds a "Using Combine with Amplify" section to the Project Setup tab for iOS. This subsection does not appear for Android or JS
• It separates Amplify API code samples into "Listener (iOS 11+)" and "Combine (iOS 13+)" sections. This should allow customers easy access to both styles, since it's fully plausible that a customer may wish to mix & match styles in the same project

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[harrysolovay]

@palpatim in the future, in the case that a given PR should not be merged until a condition is met (in this case the release of 1.10), please submit the PR in draft mode.

[palpatim]

1.1 is released. This PR is now pending @drochetti review.

[drochetti]

This was massive! Thanks 👍👍
I scanned all code snippets just looking for major mistakes but didn't find any.

I'll keep my eyes open for customer feedback once we announce it and will fix any problems if needed.