Add Federal Website Index filtering process diagram

[Copilot]

This PR adds a comprehensive visual diagram illustrating the 6-step Federal Website Index filtering process described in pages/index_narrowing_steps.md.

What's Added

New diagram file: pages/index_narrowing_process_diagram.md containing:

• Interactive Mermaid flowchart showing the complete filtering pipeline from ~35,000 URLs down to ~10,000 URLs
• Color-coded visualization with different node types (start, process, engine, end) for easy understanding
• Data snapshot references linking to actual CSV files and analysis reports at each step
• Process summary table with step numbers, descriptions, input/output counts, and key files
• Alternative simple text view for accessibility and users who prefer ASCII-style diagrams

Enhanced navigation: Updated both pages/index_narrowing_steps.md and pages/technical_details-copy.md to prominently link to the new visual diagram.

Process Visualization

The diagram illustrates how the Federal Website Index is refined through these steps:

1. Deduplicate (35k → 31k URLs)
2. Apply Ignore List (31k → 29k URLs) - removes non-public sites
3. Remove Non-Federal Domains (29k → 27k URLs) - removes non-.gov and expired domains
4. Remove Inactive Sites (27k → 14k URLs) - filters out inactive sites and data files
5. Deduplicate Final URLs (14k → 12k URLs)
6. Deduplicate Final Websites (12k → 10k URLs)

The visual representation makes it much easier to understand the data flow, volume reduction at each stage, and the relationship between different snapshots and analysis reports.

Fixes #22.

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