feat(website): add /sponsorship/ landing page

Adds a dedicated sponsorship page at /sponsorship/ built from the Jinja2
template, with hero stats, tier cards, and CSS. Updates the index.html
sponsor sidebar link to point to /sponsorship/ instead of the GitHub
SPONSORSHIP.md. Adds the URL to the sitemap and test fixtures.

Also renames .impeccable.md to DESIGN.md.

Co-Authored-By: Claude <noreply@anthropic.com>