* Add prev/next navigation to blog posts, reduce index gap
* Add download/GitHub CTA to all blog posts via layout
* Track blog post slug in PostHog download/GitHub click events
- Delete docs-site/ (superseded by web/app/docs)
- Add posthog-js with Vercel reverse proxy at /cdata to bypass adblockers
- Track pageviews (SPA-aware), download clicks (hero/navbar/mobile_drawer),
and GitHub link clicks (hero/navbar/mobile_drawer/footer)