Cache-bust CSS/JS assets and add intrinsic SVG dimensions on hero

[ammaarreshi]

Summary

• Cache-bust stylesheet & script URLs (zensical.toml, scripts/lint_docs.py): Append ?v=2 to extra_css and extra_javascript entries so returning browsers with cached stylesheets from before PR Polish gemma.ai hero, search, navigation, and callout design #7 don't serve stale home.css/nav.css against the updated HTML.
• Intrinsic SVG dimensions (overrides/home.html): Add explicit width/height HTML attributes (15x15 and 16x16) on the hero search and button <svg> icons so they never expand to full viewport width if CSS is still loading or cached.