Rework the docs landing page and navigation

[muscariello]

• Add an “Introduction / Why SHADI / Threat Model / Quick Start / Core Concepts / Examples”
• Separate “operator docs” from “developer docs” from “agent docs”.
• Add a dedicated “Sandbox Sessions and Change Tracking” page once git snapshots land.
• Add a dedicated “Telemetry and Observability” page once core OTel lands.
• Tighten docs/index.md and mkdocs.yml first, then expand the content pages.