docs: add Star History chart to end of READMEs (#294)

Appends a star-history.com chart (light/dark via <picture>) to the end of
the English, Spanish, and Korean READMEs. Heading follows each file's
convention: translated in Spanish ("Historial de estrellas"), English
elsewhere.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>