Restructure root README with clear user paths

[kmcginnes]

Description

Replace the "Learn More", "Getting Started", and "Connections" sections in the root README.md with two focused sections:

Get Started

Two clear paths based on where the user is:

• New to Graph Explorer? → Link to the getting-started tutorial
• Ready to connect your own database? → Link to the guides (connection + deployment)

Documentation

A clean map of the full docs structure for users who know what they are looking for: features, guides, references, development, contributing, roadmap, changelog.

Additional Scope

• Review and align docs/README.md (the docs index page) so the two index pages complement each other rather than overlap
• Keep the existing product intro and screenshots at the top — those are working well
• Remove the "Connections" section (its content is product description that already lives in the intro)

Dependencies

• Restructure getting-started page into a hands-on tutorial #1716 (tutorial must exist before we can link to it)

Related Issues

• Part of Update getting started docs #1097