Explain task categories and refine dataset documentation

Explain Accuracy and Brier before the leaderboard

Simplify distribution chart and add numbered evaluation notes

Update README.md

Fit category comparison table to dataset card width

Refine dataset card with task chart and unified category leaderboard

Update README.md

Update README.md

Simplify MMDM evaluation split, leaderboard and dataset card

Add MMDM dev/test, dataset card and hard/soft leaderboards

initial commit