Add screenshot carousel, polish typography and theme styling

[kmcginnes]

Description

Builds on the Starlight docs foundation with a polished landing page and refined theme:

• Screenshot carousel on the landing page cycling through Graph Explorer, Schema Explorer, and Data Explorer views
• Feature cards using Lucide icons with a stacked layout
• Typographic scale using a minor-third (1.2) ratio with tiered heading weights and negative letter-spacing
• Theme refinements: font smoothing, button glow on hover, frosted nav bar, sidebar styling (uppercase group labels, accent-tinted active state, no indentation), tab styling, content link hover underlines
• CSS split into tokens.css (design tokens) and custom.css (component overrides) for maintainability
• Graph background hero animation with mouse-reactive drift and horizontal overflow fix

How to read

Start with tokens.css for the design system, then custom.css for component overrides. The landing page lives in src/pages/index.astro and delegates to Hero.astro, GraphBackground.astro, ScreenshotCarousel.astro, and FeatureCard.astro.

Validation

• Visual inspection of landing page and inner pages in both dark and light modes
• No horizontal scroll at any viewport width
• pnpm checks passes

Related Issues

• Related to Polish docs site with custom theme, interactive hero, and link fixes #1763
• Related to Scaffold Starlight docs site and migrate content #1759

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.