Refactor database connection and update documentation

[NikhilSehgal123]

Refactors the codebase to enhance security, flexibility, and robustness, and updates documentation for better setup guidance.

• Abstracts Database Connection: Introduces a new db_connection.py file that abstracts the database connection logic using environment variables, reducing repetition and enhancing security.
• Improves Error Handling: Implements try-except blocks in vector_database.py for better error handling around database operations, increasing the robustness of the code.
• Updates Environment Configuration: Modifies vector_database.py to utilize environment variables for database connection parameters, removing hard-coded values.
• Enhances Documentation: Updates the README.md file with detailed instructions on setting up environment variables for database connection and enabling the pgvector extension in Azure PostgreSQL.
• Increases Flexibility in Usage: Modifies usage.py to dynamically retrieve vector dimensions from an environment variable, allowing for more flexible vector operations.

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