minor docs styling updates

[skarim]

• Add proper meta tags for social sharing previews
• Fix stack diagram styling to invert colors properly in Safari dark mode

[Copilot]

Pull request overview

Updates the documentation landing page visuals and improves social-sharing metadata for the Astro/Starlight docs site.

Changes:

• Replaces the static stack diagram SVG asset usage with an inline StackDiagram Astro component.
• Adds Open Graph/Twitter meta tags (and updates the site description) for richer link previews.
• Adds a social card image asset and removes the old stack-diagram.svg file.

Show a summary per file

File	Description
docs/src/content/docs/index.mdx	Swaps embedded diagram image markup for a reusable StackDiagram component.
docs/src/components/StackDiagram.astro	Introduces an inline SVG diagram component with theme-aware styling.
docs/src/assets/stack-diagram.svg	Removes the old static SVG asset no longer referenced.
docs/public/github-social-card.jpg	Adds a social preview image referenced by OG/Twitter meta tags.
docs/astro.config.mjs	Updates site description and adds OG/Twitter metadata tags (and removes robots noindex).

Copilot's findings

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Comments suppressed due to low confidence (1)

docs/src/components/StackDiagram.astro:61

• The wrapper hard-codes text colors (#1f1f1f / #ffffff), but the docs theme already defines --sl-color-text (and switches it by data-theme). Using the theme variable here will keep the SVG aligned with future palette changes and avoid mismatches if additional themes are added.

<style>
  .stack-diagram-wrapper {
    color: #1f1f1f;
  }
  :root[data-theme='dark'] .stack-diagram-wrapper {
    color: #ffffff;
  }

• Files reviewed: 3/5 changed files
• Comments generated: 3