docs: improve README for ingestion pipeline clarity and presentation readiness

[ZeroQuest]

Overview

This PR improves the project README to better reflect the current state of the ingestion pipeline after Dockerization and recent ETL enhancements.

The goal is to make the repository easier to understand, run, and present for both technical reviewers and the project demo.

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Changes

• Reorganized README into clearer sections (Features, Architecture, Pipeline Flow)
• Clarified ingestion pipeline stages (Reader → Validator → Cleaner → Loader)
• Added explicit reject handling flow (stg_rejects)
• Improved repository structure documentation
• Cleaned up configuration and setup instructions
• Improved consistency in terminology (staging / bronze layer alignment)
• Reduced ambiguity in pipeline behavior description

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Motivation

The previous README was functionally correct but lacked clarity in:

• Separation of pipeline stages
• Explicit data flow visualization
• Presentation-ready structure for a short demo
• Alignment with Bronze/Silver/Gold architecture direction

This update improves readability and better communicates system design decisions.

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Current System Context

The system now supports:

• Ingestion of CSV, JSON, and API-based data sources
• Schema validation and rule-based data filtering
• Storage of validated data in PostgreSQL staging tables (stg_*)
• Capture of invalid records in stg_rejects with reason and payload
• Config-driven ETL execution model
• Fully Dockerized runtime environment

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Future Alignment

This README prepares the project for future extensions including:

• Silver layer (cleaned and standardized datasets)
• Gold layer (analytics-ready fact tables)
• Multi-domain ingestion (weather, traffic, energy)
• Downstream analytical workloads and correlations

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Verification

No functional code changes were introduced in this PR.

Validation performed:

• Pipeline executes successfully in Docker
• All README instructions are reproducible
• Documentation matches current system behavior

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Notes

This PR is documentation-focused and does not modify pipeline logic.