chore: getting started doc for iOS

[abdulraqeeb33]

Description

One Line Summary

Add a Getting Started guide for running and testing the Red App (SwiftUI sample app).

Details

Motivation

New developers on the team had no concise onboarding document for building and running the iOS sample app locally. This PR adds a GettingStarted.md that walks a Senior SWE through setup, running the app, and understanding every testable feature — reducing ramp-up time.

Scope

• Added: GettingStarted.md at the project root with prerequisites, three alternate ways to open the project in Xcode, App ID configuration, a full feature reference table, and common troubleshooting tips.
• Added: docs/assets/red-app-screenshot.png — a screenshot of the Red App embedded in the guide for visual context.
• No SDK source code, build configuration, or test changes.

Testing

Unit testing

N/A — documentation-only change; no code modified.

Manual testing

Verified the markdown renders correctly on GitHub (headings, tables, image embed, code blocks) and confirmed all referenced file paths (iOS_SDK/OneSignalSDK.xcworkspace, OneSignalSwiftUIExample scheme, OneSignalService.swift) are accurate against the current repo structure.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

None of the above apply — this is a documentation-only change.

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
  • Adds a single Getting Started guide and its supporting screenshot asset.
• Any Public API changes are explained in the PR details and conform to existing APIs
  • N/A — no API changes.

Testing

• I have included test coverage for these changes, or explained why they are not needed
  • Documentation-only; no testable code paths introduced.
• All automated tests pass, or I explained why that is not possible
  • No tests affected.
• I have personally tested this on my device, or explained why that is not possible
  • Verified markdown rendering and file-path accuracy; no runtime behavior to test on device.

Final pass

• Code is as readable as possible.
  • Markdown is concise, uses tables for scanability, and provides three clearly labeled options for opening the project.
• I have reviewed this PR myself, ensuring it meets each checklist item

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This change is 

[nan-li]

nice