fix(site): update landing page links to docs site and improve docs design

- Fix nav "Docs" link: pkg.go.dev -> /docs/
- Fix blog card links: GitHub blob -> /docs/blog/
- Fix CTA "Getting Started": GitHub blob -> /docs/getting-started/
- Fix footer links: add Documentation, Getting Started, keep Go Reference
- Fix benchmark methodology link -> /docs/reference/benchmarks/
- Redesign docs SCSS: gradient headings, active sidebar indicator,
  rounded code blocks with brand tint, polished tables, better
  typography, subtle scrollbars, improved dark/light themes

Author: dndungu