Data blog posts: add a 'How to read the results' definitions table before the first results table

Readers stumbled on undefined terms ('win rate', 'unrestricted bowl', 'popularity kept') in a results table. What fixed it, datathon-report style: a short subsection right before the results with a Measure | Definition | Example table. Each definition states the source column and sample size (FiveThirtyEight winpercent, ~269k matchups), the formula with thresholds and exclusions, and each example works a real value through it (59.5% / 76.3% = 78.0%). Follow with one bold line explaining how to interpret the headline metric (78% kept = about 22% lower average win rate). Verify every example number against the results file before publishing.