Add data-source validation links for all 36 problem statements

[AshayK003]

Background

Each problem statement includes a "Data Sources" section with links to publicly available datasets. Some links may have gone stale since initial curation.

Task

Validate every data source link across all 4 tracks:

1. Check each URL in the ## Data Sources sections of every .md file
2. For broken links, find working alternatives or archive.org snapshots
3. For each validated link, append ✅ Verified: YYYY-MM-DD as a comment in the file
4. Open a PR with [data-validation] in the title

Files to check

• global-south-impact/*.md
• us-civic-tech/*.md
• india-impact/*.md
• rapid-prototype/*.md

Expected output

A single PR that touches all files with broken links fixed and verified links marked.

Skills needed

• Bash/Python for link checking (optional)
• Basic markdown editing

Estimated effort: 2-3 hours