feat: add section navigation and embedded Archify diagrams

docs/contribution/contribute.md

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+---
+title: Contribute
+description: Contribute evidence, proposals, and documentation changes to Eldorado Powered By AI.
+slug: /
+---
+
+Use this section to contribute to **Eldorado Powered By AI**.
+
+- Distinguish evidence, proposals, open questions, and agreed decisions.
+- Do not present example diagrams as the actual organization or architecture.
+- Use a pull request for source changes; automated checks must pass before publication.
+
+## Maintain the portal
+
+[Update and publish the site](/docs/maintain/) or consult the [configuration reference](/docs/reference/).
+
+## Create diagrams
+
+[Author and embed an Archify diagram](./diagrams.md).