Racket v9.1: language-specific docs, curved syntax arrows & more

Racket v9.1 introduces language-family specific documentation navigation, first available with Rhombus, enabling tailored documentation experiences. DrRacket now features curved syntax arrows that show left or right displacement, and the for form and its variants include an #:on-length-mismatch specifier. Other updates bring improvements to the color scheme chooser, portable error mapping, and several technical enhancements.