Revise README and handle error messages in ChromaDB

[Maclenn77]

This pull request introduces several improvements to GnosisPages, focusing on enhanced documentation, better environment configuration, more robust agent prompt management, and improved error handling for database queries. The changes make the app easier to set up and use, clarify its architecture and use cases, and ensure more reliable user feedback in case of misconfiguration.

Documentation & Setup Improvements:

• Major rewrite and expansion of the README.md to clarify the product's purpose, architecture, data flow, setup instructions, and deployment process. It now includes detailed explanations, usage scenarios, and a project structure overview.
• Added a sample environment file .example.env with placeholders for the OpenAI API key and system prompt path, making it easier for users to configure the application.

Agent & Prompt Handling:

• Added logic in gnosis/agent.py to load the system prompt from a file path or environment variable, with a sensible default fallback. This allows for easier customization of the assistant's behavior.
• Updated the agent initialization to inject the loaded system prompt into the ChromaDB search tool.
• Added a new demo prompt file (prompts/demo-prompt.md) with clear guidelines for the recruitment assistant use case.

Error Handling & State Management:

• Improved error messages in gnosis/search.py to handle missing API keys, inaccessible collections, and empty databases, providing clear user feedback in each case.
• Ensured that the ChromaDB instance is stored in the Streamlit session state, preventing unnecessary re-initialization and improving app stability.

Summary of most important changes:

Documentation and Setup

• Comprehensive rewrite of README.md to clarify use cases, architecture, setup, and deployment, making onboarding easier for new users.
• Addition of .example.env to guide users in configuring environment variables.

Agent Prompt Handling

• System prompt can now be loaded from a file or environment variable, with a fallback default, increasing flexibility for different deployment scenarios. [1] [2]
• Added a recruitment-focused demo prompt in prompts/demo-prompt.md for improved LLM guidance.

Robustness and Error Handling

• Improved error messages in the ChromaDB search tool for missing API keys, inaccessible collections, and empty databases, resulting in better user experience.
• ChromaDB instance is now managed in Streamlit session state, preventing repeated instantiation and potential bugs.