Clean up old docs directory and update README links

[kmcginnes]

Parent PRD

• Part of Migrate documentation to Starlight (Astro) #1754

What to build

Remove the old docs/ directory (now that all content lives in packages/docs/) and update the root README to point to the deployed documentation site.

End-to-end, this means:

1. Delete the docs/ directory and all its contents
2. Create a new docs/README.md with a single redirect message pointing to https://aws.github.io/graph-explorer/ and a note that source lives in packages/docs/
3. Update root README.md links:
   • ./docs/getting-started/README.md → https://aws.github.io/graph-explorer/getting-started/
   • ./docs/guides → https://aws.github.io/graph-explorer/guides/
   • ./docs/development.md → https://aws.github.io/graph-explorer/references/development/
   • ./docs → https://aws.github.io/graph-explorer/
4. Leave root README.md images and images/ directory untouched

Acceptance criteria

• Old docs/ directory is removed (except for the new redirect README)
• docs/README.md points users to the deployed site and source location
• Root README.md links point to https://aws.github.io/graph-explorer/ URLs
• Root README.md images still render correctly on GitHub
• No broken links remain in the root README

Blocked by

• Blocked by Migrate documentation content to Starlight #1756
• Blocked by Add GitHub Actions workflow for docs deployment to GitHub Pages #1757

User stories addressed

• User story 8 (canonical documentation URL)