feat: enhance devcontainer and refactor API key handling

[kaushalpowar]

This PR introduces several enhancements to the Talk to PDF application, focusing on improving the development environment and refining the API key handling process.

Development Environment Enhancements

The .devcontainer/devcontainer.json file has been significantly improved to provide a more comprehensive and user-friendly development environment. Key changes include:

• More descriptive name: The container name is now "Python 3 Development Environment" for better clarity.
• Explicit customizations: The customizations section is now clearly structured, separating codespaces settings from VS Code settings.
• Python settings: Added VS Code settings for Python interpreter path, linting, formatting (using Black), and editor preferences (rulers, trailing whitespace).
• Essential extensions: Included essential extensions like ms-python.black-formatter, njpwerner.autodocstring, streetsidesoftware.code-spell-checker, eamodio.gitlens, and GitHub.copilot for enhanced productivity.
• Improved package installation: The updateContentCommand is now more robust, handling packages.txt and requirements.txt separately and installing essential packages like black, pylint, and pytest.
• Application startup: The postAttachCommand now includes --server.address 0.0.0.0 to allow access from other devices.
• Port attributes: Added more details to the portsAttributes section for better clarity.
• Security enhancements: Added remoteUser, features (git, github-cli), runArgs (for ptrace and seccomp), and mounts (for SSH keys) for improved security and git operations.

API Key Handling Refactoring

The API key handling has been completely refactored in 0_🔌API_KEY.py to improve security and user experience:

• API Key Class: Created an APIKeyConfig class to encapsulate API key management logic.
• Key Validation: Added validate_api_key method for basic key format validation.
• Secure Storage: API keys are now stored securely in the session state.
• Error Handling: Improved error handling for invalid API keys.
• Modular Design: The code is now more modular and readable.
• Clearer UI: The user interface for API key input is more user-friendly.

Documentation Improvements

The README.md and LEARN.md files have been significantly improved to provide more comprehensive documentation:

• Structured Content: Improved organization with clear headings and sections.
• Detailed Explanations: Added detailed explanations of features, installation, usage, and architecture.
• Troubleshooting: Included a troubleshooting section with common issues and solutions.
• Contributing Guidelines: Added contributing guidelines for potential contributors.
• Badges: Added badges to showcase the project's technologies and license.

These changes enhance the overall user experience and developer workflow, making the Talk to PDF application more robust and user-friendly.