Write the Documentation the Codebase Is Missing

Background: Native [iOS / Android] app. Documentation is [nonexistent /
stale / scattered]. New engineers take [time] to become productive. Here
is the project structure and the code that most needs explaining:

[Paste structure and the relevant subsystem]

Use case: Write the documentation this codebase should have had—the kind
that answers what a newcomer actually asks, not what's easy to generate.

Implementation details:
- Explain decisions and their reasons, not just structure; a reader can
  see the folder names already
- Cover: how to get the app running locally, the architecture and why it
  was chosen, the subsystems with non-obvious behavior, and the gotchas
  that will otherwise be learned by breaking something
- Write down what is deliberately not done and why—the absent choices are
  usually the most expensive knowledge to reacquire
- Mark anything you inferred rather than read, so a human can verify it
- Keep it maintainable: prefer documentation that goes stale slowly

Deliverable: A README covering setup and orientation, an architecture
document with the decisions and their rationale, and inline documentation
comments for the subsystem above—plus a list of the things you could not
determine from the code that a human needs to fill in.