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

Tinybird restructured its documentation around developer workflows with a maximum two-level sidebar hierarchy. The new architecture organizes content as quickstart, development workflow, core concepts, actions, and guides—with SDK examples (TypeScript, Python) integrated throughout rather than isolated in reference sections. The redesign also eliminates the Classic/Forward toggle in favor of a method selector, allowing readers to choose their preferred tools once and see consistent examples throughout.