docs: add light/dark mode toggle and improve documentation colors

Add a three-state theme toggle (light/dark/auto) to doc pages with
localStorage persistence and FOUC prevention. Align doc color palette
with the homepage purple/indigo theme. Switch syntax highlighting to
CSS classes (noClasses=false) with Tango (light) and Dracula (dark)
Chroma themes. Fix whitespace underline artifacts in code blocks.

Author: fhussonnois