Add folder descriptions to README with links to DART, LISSY, and data availability visuals

[Copilot]

Overview

This PR updates the main README.md to include a new "Repository Structure" section that provides clear descriptions and navigation links to three key folders in the repository:

• DART - Data Analysis and Reproducibility Toolkit
• LISSY - Remote-execution system documentation and tutorials
• Data Availability Visuals - Heatmap visualizations showing data availability across countries and years

Changes Made

Added a new "Repository Structure" section between the "Objectives" and "Methodology" sections that includes:

DART Folder

Link to ./DART with a description of the Data Analysis and Reproducibility Toolkit files, including validation scripts, methodological notes, and visualization outputs for analyzing income indicators and generating cross-national poverty metric comparisons.

LISSY Folder

Link to ./LISSY with comprehensive information about the LISSY remote-execution system documentation, highlighting:

• Tutorial materials for getting started
• Code examples in R, SAS, SPSS, and Stata
• Best practices for LISSY job submission

Data Availability Visuals Folder

Link to ./analysis/data-availability/visuals with a description of the heatmap visualization tools that show which countries have data available for specific time periods, helping researchers identify gaps and opportunities for cross-national analysis.

Why This Matters

The new section improves repository navigation and helps users quickly locate important resources. By providing clear descriptions of each folder's purpose, researchers can more easily understand the repository structure and find the tools and documentation they need for their analysis work.

Fixes the documentation gap by making key folders more discoverable and their purposes more explicit.

Original prompt

Add folders description to the readme

Update the docs info so it includes the link to heatmaps (availablity visuals), DART and LISSY folders.

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