☂️ Refactor Flavors page

[atsansone]

Update the Flavors page accounting for the following issues:

• [ ] #8743
• [ ] #8746
• [ ] #8238
• [ ] #9309
• [ ] #9359
• [ ] #9360
• [ ] #9361
• [ ] #9988
• [ ] #9993
• [ ] #10309
• [ ] #10713

This covers:

• [ ] Updating all screenshots for Xcode 15
• [ ] Updating iOS Xcode project configuration section
• [ ] Adding a sample app to the page
• [ ] Adding unique icons to each app flavor
• [ ] Adding docs for the appFlavor TL constant

https://docs.flutter.dev/deployment/flavors/

TODOs for flavors page:

• [ ] Add/reference an app sample with free/paid flavors.
• [ ] Update screenshots to display free and paid flavors.
• [ ] Update page to better follow google developer documentation style and formatting guidelines
• [ ] Organize all iOS content under a single header
• [ ] Better clarify meaning of "desired flavors" in Android documentation
• [ ] Better format explanation of options in build.gradle
• [ ] Full copy edit of page

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Covers:

• [ ] https://github.com/flutter/website/issues/8743
• [ ] https://github.com/flutter/website/issues/8746
• [ ] https://github.com/flutter/website/issues/8238
• [ ] https://github.com/flutter/website/issues/9309