docs: README simplify migration advisory and update links

[nan-li]

Description

One Line Summary

README - simplify migration advisory and update links.

Details

• Remove stale v5 feature parity warning now that v5 is stable. Replace with concise migration guide reference.
• Fix image embedding to use GitHub-hosted URL (previously would not display)
• Update documentation links (broken links)
• Update supported iOS versions

Motivation

Clearer README in this repo

Testing

Unit testing

N/A

Manual testing

Visually checked

Affected code checklist

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

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

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This change is