docs: complete technical documentation and security architecture mapping (#15)

[Mike031289]

This PR delivers the complete technical documentation package for the ToDo & Co application. It structures the onboarding process, maps the security and authorization system, and upgrades the project entry point.

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Key Deliverables Included

1. Root README.md Overhaul

• Context & Goals: Documented the application migration journey (from Symfony 3.1/PHP 5.5 to Symfony 3.4/PHP 7.4).
• Fast-Track Guide: Provided streamlined instructions for database setup, fixture loading, and local server initialization.
• Documentation Mapping: Linked sub-guides (CONTRIBUTING.md and doc/security.md) to establish a clean entry-point.

2. Technical Security Guide (doc/security.md)

• Authentication & Firewalls: Mapped session behaviors and user provider loading mechanics.
• Role Hierarchy Diagram: Explicitly detailed the inheritance structure (ROLE_ADMIN inheriting from ROLE_USER).
• Access Control Matrix: Documented route protections (security.yml) and complex business rules managed by our custom TaskVoter (including the specific ROLE_ADMIN exception for anonymous task cleanup).
• Demo Accounts Registry: Listed standard test accounts (admin, user, anonyme) to help with quick validation.

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Acceptance Criteria Checklist

• Completely modernize the root README.md.
• Map and document the application's role hierarchy.
• Detail route-level protection rules and Symfony Voter behaviors.
• Provide standard test credentials and database fixture initialization commands.
• Ensure correct Markdown rendering and documentation linkages.

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Verification

1. Static Analysis & Rendering: Verified overall Markdown layout directly in the GitHub preview panel to ensure perfect readability.
2. Setup Validation: Standard setup instructions have been successfully tested against a clean database installation.

Close #15

[codacy-production]

Up to standards ✅

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