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.

.gitignore

@@ -0,0 +1,15 @@
+# Specific to this project
+.github/instructions/
+images/
+src/
+scripts/
+
+# Environment files
+.env
+.env.local
+.env.*.local
+.vscode/
+.idea/
+.DS_Store
+dist/
+build/