Document First Flags callbacks and CACHED evaluation reasons

[typotter]

What does this PR do? What is the motivation?

Document how client applications can react to the first accepted flag configuration and interpret CACHED evaluation reasons during startup. The guide covers Android, Apple platforms, and Dart/Flutter, with examples and cleanup guidance.

Merge readiness

• Ready for merge

Keep this PR in draft until the Apple and Dart/Flutter release requirements are confirmed. The guide is marked draft: true and private: true; review visibility and navigation before publication.

• Android: First Flags and CACHED verified in published dd-sdk-android-flags 3.15.0, including its Maven sources archive. Neither appears in the previous 3.14.1 sources.
• Apple: latest verified release 3.19.0 contains neither feature. CACHED is merged after that release; First Flags remains in PR Translation PR #3269. No minimum version is claimed.
• Dart/Flutter: published datadog_flags 1.1.1 and datadog_flags_flutter 1.1.0 archives contain neither feature. First Flags examples follow the direct-client API in PR Jhotta/adding jp message to insert example links method #1204. No minimum version is claimed.

AI assistance

Used Codex for source and release research, documentation drafting, and validation.

For Datadog employees:

• The branch follows the required <name>/<description> convention.
• Automated /review has not been requested by this change.

Additional notes

Validated the Kotlin, Swift, and Dart examples against the applicable published or PR source. No new snippet compilation or SDK runtime tests were performed. Vale reports no findings in the guide and no new findings compared with the existing three client pages. Guide references, code fences, whitespace, and repository commit hooks pass. Local Hugo build was not run; preview validation remains pending.

The existing cached-assignment delivery PR #40213 describes CDN fallback and does not duplicate this local-storage and callback guide. A documentation ticket key has not been supplied.

Release/source references: Android 3.15.0, iOS 3.19.0, iOS First Flags, Flutter First Flags, Dart package, Flutter package.

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/typo/docs-first-flags/feature_flags/guide/first_flags

Modified Files

• https://docs-staging.datadoghq.com/typo/docs-first-flags/feature_flags/client/android
• https://docs-staging.datadoghq.com/typo/docs-first-flags/feature_flags/client/flutter
• https://docs-staging.datadoghq.com/typo/docs-first-flags/feature_flags/client/ios