docs: fix broken App Transport Security links and update ATS documentation

[supunyasanthaofficial]

Summary

This PR fixes broken internal anchors and modernizes the App Transport Security (ATS) guidance across documentation:

1. Broken anchors in images.md & network.md :

• publishing-to-app-store.md#1-enable-app-transport-security (anchor was removed years ago).
• integration-with-existing-apps.md#test-your-integration (anchor does not exist in modern tabbed integration guide).
• Obsolete 2015 CocoaKeys Apple documentation URL.

2. Updated ATS guidance in network.md :

• Mentions NSAppTransportSecurity and links directly to Apple's modern reference.
• Recommends using NSExceptionDomains for specific domains rather than disabling ATS globally (NSAllowsArbitraryLoads), noting App Store review justifications.
• Links to Apple's official guide on Preventing Insecure Network Connections.

3. Backported :

• Backported to all versioned docs (version-0.77 through version-0.87).

Test plan

• Run ESLint on modified files: passed.
• Run Docusaurus production build: verified all 6 broken ATS anchor warnings were eliminated.