[Documentation]: Architecture learning journey references outdated user news resource use case

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Page URL (type "NEW" for a new page suggestion)

https://github.com/android/nowinandroid/blob/main/docs/ArchitectureLearningJourney.md

### What's the documentation problem or suggestion?

## Problem

`docs/ArchitectureLearningJourney.md` appears to describe an older For You feed flow.

The document references `GetUserNewsResourcesUseCase`, but that class no longer exists in the current codebase. The current implementation uses:

- `ForYouViewModel.feedState`
- `UserNewsResourceRepository.observeAllForFollowedTopics`
- `CompositeUserNewsResourceRepository`, which combines `NewsRepository` data with `UserDataRepository` data

The domain-layer example also uses `GetUserNewsResourcesUseCase`, so readers following the architecture learning journey may search for a class that is no longer present.

## Suggested solution

Update `docs/ArchitectureLearningJourney.md` so it matches the current implementation:

- Replace the stale `GetUserNewsResourcesUseCase` references with `UserNewsResourceRepository` / `CompositeUserNewsResourceRepository`
- Point readers to `ForYouViewModel.feedState` for the For You feed flow
- Use `GetFollowableTopicsUseCase` as the current domain-layer example

I have opened a small documentation PR for this change.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start with docs/ArchitectureLearningJourney.md and compare its For You feed and domain-layer references with ForYouViewModel.feedState, UserNewsResourceRepository.observeAllForFollowedTopics, CompositeUserNewsResourceRepository, and GetFollowableTopicsUseCase. Done means the document no longer references GetUserNewsResourcesUseCase and accurately points readers to the current implementation.

Written by the indexing model from the issue text.