Draft: group docs around SDK, MCP, CLI, and Rust routes

[JimCollinson]

Summary

• reorganize the navigation so Getting Started and How-to Guides are grouped by the four user-facing routes: SDK, MCP, CLI, and Direct Rust
• add a new Use MCP with AI Tools Getting Started page so MCP becomes a first-class entry path instead of being buried under SDK material
• move cross-route pages into Common Tasks and explicitly mark the most mixed pages as comparison or shared-task guides
• update contributor guidance so route is treated as a required second-level axis and tabs are used for languages within one route, not for switching routes

Why

The docs already have a top-level Diataxis shape, but route clarity under Getting Started and How-to has been inconsistent. This PR makes the user-facing interfaces the primary organizing axis underneath those sections while keeping Core Concepts route-neutral. The goal is to help readers stay in one path long enough to build a coherent mental model before they hop across tools.

Source Refs

• ant-sdk: bf541ccd4ae1fd3e174fb7b5bb21deef38d999ce
• ant-client: d46a73d38731a31fbd9815394fe8a2943eb38246
• ant-node: d3f5ba969b8ccf98ca0c50b661a3762aec904634

Review Focus

• confirm the new route grouping in SUMMARY.md is more navigable than the previous mixed layout
• confirm MCP is surfaced as its own user-facing route while still accurately described as using antd under the hood
• confirm the mixed-route pages moved under Common Tasks are framed clearly enough as shared-task or comparison pages
• confirm the new contributor rules are the right guardrails for future docs work