Update Starlight social config to new array schema to unblock docs build

[Copilot]

The build GitHub Actions job fails during astro build because Starlight no longer accepts object-style social config. This PR updates docs site configuration to match the current Starlight schema expected by the deployed dependency range.

• Root cause

  • docs/astro.config.mjs used legacy social object syntax ({ github: "..." }), which is rejected by current @astrojs/starlight.

• Config migration

  • Replaced legacy social object with the new array-of-links format required by Starlight.
  • Kept the same GitHub URL and user-visible behavior; only schema shape changed.

• Scope

  • Single-file, config-only change in docs site setup (docs/astro.config.mjs).

// before
social: {
  github: 'https://github.com/Saaalil/DocRagGraph',
},

// after
social: [
  { icon: 'github', label: 'GitHub', href: 'https://github.com/Saaalil/DocRagGraph' },
],