feat: Enhance Talk to PDF app with improved error handling and new features

[kaushalpowar]

This PR introduces several significant improvements to the Talk to PDF application, focusing on enhancing the user experience, improving error handling, and adding new features.

Key Improvements:

• API Key Handling: The API key handling has been significantly improved. The process is now more user-friendly, with clearer instructions and more informative error messages. A validation step is included to check the API key before proceeding.
• Robust Error Handling: The application now includes more comprehensive error handling to prevent unexpected crashes and provide users with helpful information when issues occur. Detailed error messages are displayed to the user, and logging is implemented to aid in debugging.
• Code Structure and Readability: The code has been restructured to improve readability and maintainability. Functions and classes have been introduced to separate concerns, making the code easier to understand and modify. Comprehensive docstrings have been added to explain the purpose and functionality of each module and function.
• UI Enhancements: The user interface has been updated to provide better feedback to the user. Progress indicators and more informative messages improve the overall user experience.
• Multiple PDF Uploads: The application now supports uploading multiple PDF files simultaneously, allowing users to process multiple documents at once.
• Model Selection: Users can now choose between different OpenAI models (gpt-3.5-turbo or gpt-4) based on their needs and preferences.
• Temperature Adjustment: A slider has been added to allow users to adjust the temperature parameter, controlling the randomness of the model's output. This provides more control over the creativity and focus of the generated responses.
• Progress Indicator: A progress indicator has been added to show the progress of document processing, providing users with feedback during this step.
• File Validation: Validation has been added to check the maximum file size and number of files uploaded, preventing issues caused by excessively large or numerous files.

Detailed Changes:

• Added a AppConfig dataclass to manage application settings.
• Implemented validate_api_key function for API key validation.
• Introduced handle_api_key_submission function to handle API key submission and state transitions.
• Created render_api_key_input function to render the API key input form.
• Refactored sidebar content into a SidebarManager class.
• Created DocumentManager class to handle document loading and processing.
• Created QueryEngineManager class to manage query engine setup and configuration.
• Updated the PDF upload page to handle multiple files and model selection.
• Improved the chat interface to handle streaming responses and provide better error messages.
• Added logging throughout the application to track events and debug issues.

This PR significantly improves the robustness, usability, and functionality of the Talk to PDF application. The added features and improved error handling make it a more reliable and user-friendly tool.