Fix stale artifact IDs in quick-start docs

[kirich1409]

Summary

• README.md and docs/getting-started.md both referenced incorrect Maven artifact coordinates that were missing the featured- prefix
• Fixed four artifact IDs: core → featured-core, datastore-provider → featured-datastore-provider, sharedpreferences-provider → featured-sharedpreferences-provider, firebase-provider → featured-firebase-provider
• Fixed Maven Central badge URL in README.md (same core → featured-core mismatch)
• All other quick-start content verified against source: class/function names, ConfigValues constructor, FlagRegistry.register(), FeatureFlagsDebugScreen, SharedPreferencesProviderConfig, SPM product name FeaturedCore, iOS xcconfig steps, Gradle plugin ID — all accurate

Note on task brief: The brief stated "defaultLocalProvider() on JVM now returns JavaPreferencesConfigValueProvider (just merged)". The actual origin/main source for JVM still returns InMemoryConfigValueProvider with a TODO(#66) comment. No docs reference JVM provider behaviour in the quick-start, so there is nothing to fix there — but the task description appears to describe work that has not yet landed on main.

Test plan

• Verify artifact coordinates resolve on Maven Central: dev.androidbroadcast.featured:featured-core, featured-datastore-provider, featured-sharedpreferences-provider, featured-firebase-provider

Closes #161

🤖 Generated with Claude Code

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