Simplify migration recipe

[dturner]

This commit refactors the step-by-step migration recipe into a more direct, atomic "before and after" example.

The previous start and stepX packages have been removed and replaced with atomic/begin and atomic/end packages. These new packages demonstrate the initial Navigation 2 state and the final migrated Navigation 3 state, respectively.

Key changes include:

• Refactoring the screen content into a shared content package to reduce duplication.
• Deleting the intermediate migration step files and the legacy Navigator.
• Updating the AndroidManifest to reflect the new activity names.
• Renaming the associated instrumented test from MigrationActivityNavigationTest to AtomicMigrationTest.
• Adding a README.md to explain the new atomic structure.

[gemini-code-assist]

Summary of Changes

Hello @dturner, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the clarity and maintainability of the Navigation 2 to Navigation 3 migration guide. By transitioning from a multi-step approach to an atomic 'before and after' example, it provides a more concise and easier-to-understand demonstration of the migration process. This refactoring involves restructuring packages, centralizing shared UI components, and updating associated tests and manifest entries, ultimately simplifying how developers can learn and apply the migration steps.

Highlights

• Migration Recipe Refactor: The step-by-step migration recipe has been refactored into a more direct, atomic 'before and after' example, simplifying the demonstration of Navigation 2 to Navigation 3 migration.
• Package Structure Update: The previous start and stepX packages have been removed and replaced with atomic/begin (demonstrating the initial Navigation 2 state) and atomic/end (demonstrating the final migrated Navigation 3 state).
• Code Duplication Reduction: Screen content has been refactored into a shared content package to significantly reduce code duplication across the migration examples.
• Cleanup of Legacy Components: Intermediate migration step files and the legacy Navigator class have been deleted, streamlining the codebase.
• Test and Manifest Updates: The AndroidManifest has been updated to reflect the new activity names, and the associated instrumented test MigrationActivityNavigationTest has been renamed to AtomicMigrationTest.
• Documentation Added: A README.md file has been added to explain the new atomic structure of the migration recipe.

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[gemini-code-assist]

Code Review

This pull request is a great refactoring of the migration recipe. It simplifies the example from a multi-step process to a clear 'before' and 'after' atomic structure, which is much easier to follow. The introduction of begin and end packages, along with the extraction of shared UI into a content package, significantly improves the clarity and maintainability of the recipe. The removal of the old step-by-step files and the corresponding migration guide document makes the repository cleaner. My main feedback is related to the instrumented test, which doesn't seem to be correctly updated to test the new begin and end activities.