Improve documentation: fix broken links, update outdated references, and add missing README files

[Copilot]

This PR addresses multiple documentation issues identified across the UniCore project, improving clarity, consistency, and completeness for developers and users.

Key Issues Fixed

Broken Links and References

• Fixed main README configuration link that pointed to non-existent ./agent_application/CONFIGURATION.md
• Updated all references to point to correct documentation locations

Inconsistent Naming

The project documentation mixed "SSI Agent" and "UniCore" terminology throughout. This PR standardizes all documentation to use "UniCore" consistently:

• Updated main README title and all references
• Fixed Docker build instructions and service names
• Updated API documentation headers
• Corrected event publisher documentation

Outdated Version References

• Updated Docker image version in quick-start guide from 1.0.0-beta.8 to 1.0.0-beta.12
• Fixed environment variable name from AGENT_API_REST_EXTERNAL_SERVER_RESPONSE_TIMEOUT_MS to UNICORE__EXTERNAL_SERVER_RESPONSE_TIMEOUT_MS

Documentation Enhancements

Added Missing README Files

Created comprehensive documentation for core agent modules that were previously undocumented:

• agent_holder: Credential holding functionality and wallet integration
• agent_verification: Credential verification and OpenID4VP support
• agent_identity: DID document management and identity components
• agent_shared: Shared utilities and configuration management
• agent_macros: Procedural macros for configuration handling

Improved Configuration Documentation

• Removed extensive commented-out sections that cluttered the configuration docs
• Added clear explanations of APPLICATION_URL vs PUBLIC_URL with production deployment examples
• Organized configuration options in logical sections with proper examples
• Added security warnings and deployment guidance

Enhanced Main README

• Added clear project description explaining UniCore's purpose and compliance with eIDAS 2.0
• Created structured "Getting Started" section with logical flow for new users
• Improved navigation between different documentation sections

Impact

These changes significantly improve the developer experience by:

• Providing clear, accurate documentation for all project components
• Eliminating confusion from inconsistent naming and broken links
• Offering comprehensive guidance for deployment and configuration
• Making the project more accessible to new contributors and users

The documentation now provides a complete and consistent reference for understanding, deploying, and using UniCore.

Original prompt

Please improve our Documentation. Remove/Update outdated documentation and add missing documentation.

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