New Tinybird Forward docs: one level of nesting and SDKs toggle

The restructured Tinybird documentation has been revamped to align more closely with the developer's workflow, featuring a streamlined sidebar with a maximum of two levels of indentation to simplify navigation. This new organization focuses on core areas like quickstart, development workflow, core concepts, actions, and guides, each designed to reflect the user's journey through the product. The documentation now integrates SDK examples for TypeScript and Python alongside CLI and HTTP calls within relevant sections, eliminating the need for a separate reference appendix. Visual enhancements have been made, including a higher-contrast dark mode and a refreshed navigation system that minimizes distractions. The previous Classic/Forward toggle has been replaced with a method selector for Tinybird CLI, TypeScript SDK, and Python SDK, allowing users to choose their preferred development path and maintain consistency across documentation. The changes aim to provide a more intuitive, efficient user experience, while the Classic docs remain available for those who still use them.