Document Android package output items

[Redth]

Adds conceptual documentation for the new .NET 11 @(ApplicationArtifact) item surface introduced by dotnet/android#11674, so custom MSBuild targets and CI workflows can discover final APK/AAB paths and metadata without recalculating package names.

Changes

• Adds a Discover Android package outputs article with examples for package signing, publish, GitHub Actions outputs, metadata enrichment, and command-line dotnet build -getTargetResult queries.
• Documents @(ApplicationArtifact), the callable GetApplicationArtifacts target, the Publish target return contract, and $(GetApplicationArtifactsDependsOn).
• Clarifies that GetApplicationArtifacts always depends on the required Build target before extension targets run, so overriding $(GetApplicationArtifactsDependsOn) cannot skip platform artifact population.
• Clarifies that targets listed in $(GetApplicationArtifactsDependsOn) update existing items with additional metadata such as $(ApplicationVersion) before GetApplicationArtifacts or Publish returns them.
• Links the new guidance from the Android build process, build item, build target, and TOC pages.

Notes

docfx was not installed locally; validated the markdown diff whitespace with git diff --check.

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Internal previews

📄 File	🔗 Preview link
docs/android/building-apps/build-items.md	docs/android/building-apps/build-items
docs/android/building-apps/build-process.md	docs/android/building-apps/build-process
docs/android/building-apps/build-targets.md	docs/android/building-apps/build-targets
docs/android/building-apps/package-outputs.md	docs/android/building-apps/package-outputs