Improve README dependency graph visualization example

[lukasniessen-bain]

Context

ArchUnitPython already supports dependency graph reporting through project_graph() / dependency_graph(), including DOT, Mermaid, D2, CSV, JSON, and HTML exports plus focus/reachability/collapse/external/self-dependency options.

The README has a concise Dependency Graph Reports section, but it currently does not show a rendered example image and the example is generic. A visual example would make the feature easier to understand at a glance.

Task

Update the README graph reporting section so it stays brief and matches the existing README style, while adding a clear visual example.

Suggested direction:

• Generate a readable dependency graph from a known Python repository, preferably a compact/familiar one such as requests or another well-known project that stays legible when focused or collapsed.
• Add the generated image to the repo assets and embed it in the README with useful alt text.
• Keep the section concise: explain the gist, show how to export a report, and mention the available formats/options without turning it into a long tutorial.
• Make sure the documented options match the current public graph API.

Acceptance Criteria

• README includes an embedded dependency graph example image.
• The example is based on a recognizable Python repository and remains readable in the README.
• The section briefly explains how to use graph reports and lists the supported formats/options.
• The wording and length fit the style of the surrounding README feature sections.

[DebanKsahu]

Assigning myself to this issue