docs: add a white Notion logo variant for dark backgrounds (#899)

Every other monochrome sponsor logo ships a _light.svg alongside the base
file so the sponsor tables in pnpm/pnpm can offer both via <picture>. Notion
was missing one, leaving it invisible on GitHub's dark theme.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: zkochan