[Cookbook] Add category structure to organize guides

[Chesars]

Summary

The Cookbook page (https://models.litellm.ai/cookbook) currently shows all guides as flat cards without any categorization. As more guides are added, this becomes harder to navigate.

Current State

All guides are displayed in a single list ordered by date, with only tags for filtering.

Proposed Structure

Organize guides into categories based on tool/use-case:

AI Coding Assistants
├── Claude Code (Quickstart, MCPs, Web Search, Non-Anthropic Models)
├── Cursor (Quickstart)
├── GitHub Copilot (Quickstart)
├── Gemini CLI (Quickstart)
└── OpenAI Codex CLI (Quickstart)

UIs & Interfaces
└── OpenWebUI (Quickstart)

Usage & Cost Management
├── AI Coding Tool Usage Tracking
└── Track usage via Custom Headers

This structure is extensible for other categories.

Benefits

• Users find guides faster (searching by tool, not scrolling)
• Scales as more guides are added
• Shows LiteLLM's ecosystem at a glance