Add prominent usage metrics to homepage metrics bar

[mmcky]

Suggestion from @DrDrij (via PR #194 review)

The metrics bar below the hero currently shows content counts (8 Lecture Series, 5+ Books, 30+ Workshops, 2 Code Libraries). DrDrij suggests adding big usage numbers like yearly readers and downloads to make the stats panel more impactful.

Potential metrics to feature:

• Yearly unique visitors/readers (analytics dashboard shows ~25-35K monthly users)
• Total downloads across code libraries
• GitHub stars across QuantEcon repos
• Countries reached

Requirements:

• Numbers must be accurate and verifiable
• Consider whether to hardcode or fetch dynamically
• The analytics dashboard at /analytics-dashboard/ may be a source for these figures

Reference: The existing metrics bar is in _layouts/home.html in the qe-metrics section.