docs: Rework Introduction page and remove Overview

[developerjamiu]

Summary

The Overview page (docs/03-overview.md) largely duplicated the Introduction page. This consolidates them into a single, sharper Introduction:

• Tightened the opening paragraph to a clear one-liner about what Serverpod is.
• Kept the server endpoint + Flutter client code example under a new heading, Write Dart everywhere.
• Added a Who Serverpod is for section (Flutter teams that want a Dart-only stack).
• Replaced the prose feature dump with a concise What's included bullet list (code gen, dev loop, ORM, client-side DB, auth, streams, deployment, logging, file uploads, scheduling, caching, web server).
• Added title and description frontmatter for meta/SEO.
• Deleted docs/03-overview.md. No inbound links pointed to it; Docusaurus auto-pagination links readers to Installation next.

Image

Test plan

• npm run build passes (no broken links).
• Render /next locally and confirm the page reads well end to end.
• Confirm the sidebar no longer shows an "Overview" entry.