docs(wiki): Sample App page is stale after multi-module sample restructure

[kirich1409]

Goal

Rewrite the wiki Sample App page to reflect the multi-module sample landed in #200 and refined in #202.

What's stale

The current page still describes the pre-restructure layout:

• Refers to a single :sample module and SampleApplication.kt/buildConfigValues().
• Instructs ./gradlew :sample:assembleDebug (the assemble target is now :sample:android-app:assembleDebug).
• Does not mention the per-feature module split (sample/feature-checkout, sample/feature-promotions, sample/feature-ui), the shared aggregator (sample/shared), or the :sample:desktop / iosApp shells.
• No mention of the per-feature *FlagsViewModels, the 3-per-feature-ConfigValues + 1-debug-aggregator wiring on Android, or the EnumDropdown in the debug UI demonstrated by the CheckoutVariant flag.

Target content

Updated page should cover:

• Module layout (3 feature modules + shared aggregator + android-app / desktop / iosApp shells).
• Build commands for each platform (:sample:android-app:assembleDebug, :sample:desktop:assemble, :sample:shared:linkDebugFrameworkIosSimulatorArm64).
• How the sample demonstrates per-module ConfigValues over a single LocalConfigValueProvider.
• How the sample wires FeatureFlagsDebugScreen with GeneratedFeaturedRegistry.all on the debug-aggregator ConfigValues.
• The Firebase opt-in flow (-PhasFirebase=true) if still relevant — verify against current sample/android-app/build.gradle.kts.

Pointer files

• sample/android-app/src/main/kotlin/dev/androidbroadcast/featured/sample/MainActivity.kt
• sample/feature-checkout/, sample/feature-promotions/, sample/feature-ui/
• sample/shared/
• sample/desktop/
• iosApp/

Definition of Done

• Sample App wiki page reflects the multi-module structure
• Build commands updated
• Cross-links to Multi-Module Setup and Debug UI pages

Related

• Multi-module sample showcasing Featured aggregator plugin #200 (multi-module sample, merged)
• Per-module ConfigValues + internal generated objects #202 (per-module ConfigValues, merged)
• Remove FlagRegistry; make FeatureFlagsDebugScreen UI-agnostic #199 (UI-agnostic debug screen)