docs(docs): add project reference and contribution documentation

[arbisoft-qaisarirfan]

🚀 Description

Add a comprehensive documentation set for the frontend project, including architecture, setup, configuration, tooling, routing, testing, deployment, and contribution guidance.

📌 Summary

This PR expands the repository documentation by introducing multiple new pages under docs/, updating the main README.md, refreshing CONTRIBUTING.md, and documenting the project docs workflow. It turns the current codebase structure and behavior into a more complete reference for onboarding, development, and maintenance.

🔧 Changes Implemented

• ✅ Added project-level reference docs covering architecture, folder structure, routing, configuration, state management, testing, and deployment
• ✅ Updated top-level documentation to better link and surface the new reference material
• ✅ Added contribution and setup guidance aligned with the current repository workflow and tooling
• ✅ Documented environment variable expectations, including the newly referenced NEXT_PUBLIC_GTM_ID

🛠️ How It Works?

1. The new documentation pages organize the codebase into focused reference areas so contributors can understand application behavior without tracing each feature manually.
2. README and contribution updates connect the main entry points in the repository to these new docs and clarify local setup and workflow expectations.
3. Deployment, tooling, and state-management docs summarize current implementation details and explicitly call out gaps or assumptions that still need confirmation.

✅ Checklist Before Merging

• Tested all relevant functionalities.
• Verified expected behavior on different use cases.
• Ensured code follows best practices and security standards.

🔗 Related Issues

Link to the associated Taiga ticket

https://projects.arbisoft.com/project/arbisoft-sessions-portal-20/us/285

📢 Notes for Reviewers

This is primarily a documentation update, so review for accuracy against the current codebase and repository workflow.

@farhan2742