Add project documentation and architecture details

- Create .gitignore to exclude specific project files and directories.
- Add CONTRIBUTING.md with guidelines for contributing to the project.
- Update README.md to reflect changes in usage section and contact information.
- Introduce 08_concepts.md outlining cross-cutting concepts in the architecture.
- Add 02_data_model.md detailing the data model and relationships.
- Implement 03_security.md to describe security measures and practices.
- Establish 08_ui_design.md for user interface design principles.
- Document quality requirements in 10_quality_requirements.md.
- Identify risks and technical debts in 11_technical_risks.md.
- Create a glossary in 12_glossary.md for project-specific terms.
- Include about-arc42.md to explain the arc42 documentation template.
- Define price calculation variables and formula in price_calculation.md.
- Introduce ADR template in adr.md for documenting architecture decisions.

architecture/about-arc42.md

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+# About arc42
+
+arc42, the template for documentation of software and system architecture.
+
+Template Version: Latest
+
+Created, maintained and © by Dr. Peter Hruschka, Dr. Gernot Starke and contributors.
+
+See [https://arc42.org](https://arc42.org).
+
+## What is arc42?
+
+arc42 is a template for documenting software architectures. It provides a comprehensive structure for capturing all aspects of a system's architecture in a consistent and understandable way.
+
+## Why use arc42?
+
+- Provides a proven structure for architecture documentation
+- Helps communicate architecture decisions to stakeholders
+- Ensures completeness and consistency
+- Is free and open source