feat: add section navigation and embedded Archify diagrams

docs/core/overview.md

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+---
+title: Explore the starting point
+description: Understand what this documentation foundation includes and what is still undecided.
+slug: /overview
+---
+
+**Eldorado Powered By AI** is the space for shaping strategy and direction for an AI-powered Eldorado.
+
+## What is included
+
+- A Docusaurus site with responsive navigation and local search.
+- An Eldorado-inspired visual theme with a dark header, gold accents, and Lato typography.
+- Automated checks on pull requests and publication to LEGO Cloud Pages after a push to `main`.
+
+## What remains open
+
+Specific AI initiatives, priorities, ownership, audience, and the roadmap remain to be defined. This starting point does not imply approved product requirements or implementation commitments.
+
+## Work on the foundation
+
+[Maintain and publish the site](./maintain.md) or [look up its configuration](./reference.md).