docs: explain import roots in the README quickstart

[LukasNiessen]

The README used project_files("src/") and metrics("src/") without explaining how that argument affects import resolution. In a src layout, filtering with in_path("src/**") alone can leave first-party imports classified as external and hide architecture violations.

Add a short import-root explanation before the first test example, linked to a detailed Setup section. Document the working-directory default, source-root versus selector behavior, examples for src, flat, and backend layouts, and the effect on dependency rules and metrics. Simplify the first cycle example by removing its redundant folder filter.

Validation:

• Executed the new README test functions against a small source-layout fixture.
• Checked file rules, metrics, and cycle detection for src, flat, and backend layouts.
• Reproduced the missed cycle with the wrong root and detected it with the documented root.
• git diff --check and ruff check src/ scripts/ pass.
• ruff format --check src/ scripts/ reports pre-existing formatting in the unchanged src/archunitpython/graph/graph_reporter.py.

README-only change; no runtime behavior changes.