docs: add architecture, external dependencies, and potential features

[Copilot]

README only had a 19-line quick-start; no documentation covered architecture, dependency transparency, or future direction.

Changes

README.md

• Added Documentation section with a navigation table linking to all doc/ pages

doc/project-overview.md

New: External Dependencies

• Required Azure OpenAI services (Chat Completion, Assistants API, Vector Stores) mapped to their exact configuration keys and runtime behavior (e.g., assistant + vector store created on startup, deleted on exit)
• Implicit network calls (Bing Search QR targets, Microsoft Learn) flagged explicitly
• NuGet dependency table with exact versions sourced from cardmaker.csproj; Windows-only platform constraint called out

New: Potential Features
Categorized backlog of extension ideas:

• Card Generation & Content — multi-deck themes, AI-generated artwork, schema validation
• Output & Integration — PDF export, Web UI, CSV/Excel
• AI & Knowledge — pre-populated vector store, multi-model support, localization
• Developer Experience — unit tests, cross-platform imaging (ImageSharp), Docker, config wizard

doc/README.md

• Updated index entry description to reflect the new sections

Original prompt

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This section details on the original issue you should resolve

<issue_title>Documentation needed</issue_title>
<issue_description>Cardmaker only has documentation, brief and high level, on the README.md. Review the project and create documentation that details the overall architecture of the solution, and helps make dependencies (e.g., model endpoints) transparent. Include a section for potential features.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Documentation needed #3

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