docs: restructure the docs site left navigation — split the single long sidebar into six icon-labelled tabs (Getting Started, Concepts, Guides, Reference, Protocol, Releases), de-emphasise the ObjectQL/ObjectUI/ObjectOS protocol branding (Protocol moved to the last tab), isolate the 238 generated reference pages into their own tab, group the Guides section (Foundations/Building/Integration/Operations/Cheatsheets), and fix the top-nav GitHub link (objectstack-ai/spec → framework). Nav-only; no doc URLs change.