Add build-time image optimization

[LJWatson]

Summary

• Replace raw <img> tags with a build-time {% image %} Nunjucks shortcode powered by @11ty/eleventy-img, generating responsive <picture> markup (WebP + original-format fallback, srcset/sizes, lazy loading)
• Applied across the About page hero image, post.html's optional feature-image slot, and 12 inline images across 7 post files — several source images were 2–3.5MB with no responsive sizing
• Alt text is enforced at build time (missing/empty alt fails the build); a { decorative: true } opt-in supports genuinely decorative images with alt=""
• Documented the shortcode's usage, options, and behaviour in docs/NOTES.md

Test plan

• npm run build succeeds with no errors
• Generated <picture> markup verified directly (webp source, fallback img, correct width/height/alt)
• Live browser check: WebP negotiation confirmed via network request, alt text confirmed via accessibility tree on About page and affected posts, at both mobile and desktop viewports
• npx linkinator ./dist --recurse re-run — all new /images/optimized/... references return 200, no regressions
• npm audit unchanged (no new vulnerabilities introduced)
• Isolated test of the shortcode's alt-text enforcement and decorative-image opt-in, both paths verified directly