docs: add sponsor page

[alixander]

Summary

• add a Sponsor D2 page explaining the public Hack Club fund and how donations are used
• add the page to the docs sidebar and route the footer Donate link through it
• add responsive, theme-aware sponsor CTA styling

Why

D2 now has a public fiscal-sponsorship fund. This makes the funding model, public ledger,
and donation path discoverable from the documentation.

Validation

• isolated Docusaurus production build for English and Korean
• Prettier 2.8.1 check
• cspell check
• alt-tag and WebP checks
• git diff --check
• local browser review at desktop width with no runtime overlay