Rewrite landing page with Google writing style

[a-github-name]

Summary

• Rewrite the landing page using Google developer writing style: direct language, sentence case, one starting command, and clear workspace access requirements.
• Update navigation, metadata, and the documentation map to describe the tasks available at each destination.
• Fix the map's internal links so they respect the GitHub Pages path prefix.

Verification

• pnpm verify
• pnpm docs:worker:test (four tests passed)
• pnpm docs:worker:dry-run
• Docs builds for / and /merekit-cli/
• Local browser inspection of the revised copy, heading structure, and primary action
• Generated-page checks for internal links and anchors

Scope and deployment

Only five documentation and theme files change. CLI behavior, package versions, dependencies, authentication, and release workflows are unchanged. Publish GitHub Pages through the existing Docs workflow and deploy the protected Cloudflare docs host from the merged revision.