docs: Implement phase 1 of the new Information Architecture

[Swiftaxe]

With this new structure we aim to:

1. Make it easy to get started
2. Make the docs predictable
3. Make information easy to find
4. Make the docs trustworthy

• Remove Overview page; move its capabilities list to Introduction
• Add a new top-level "Serverpod Academy" page after Get Started, linking to academy.serverpod.dev
• Fact-check Quickstart against the 5-minute speed-run target and tighten deviations
• Restructure the deployment section — rename to "Deploy your app", feature Serverpod Cloud as the top path, fold self-hosted options as "Custom hosting", fix the "CGP" typo in the GCP Console page title
• Write the new "Deploy to Serverpod Cloud" page
• Rename "Upgrading to pgvector support" → "Upgrade to pgvector"
• Drop Serverpod Mini from the sidebar
• Arrange the top level menu items in the desired order
• Reconcile setup-path and Cloud-status contradictions across pages (Installation says SQLite default, Quickstart requires Docker; Cloud described as four different states across four pages)