Add Mapbox to Partner Skills

[mattpodwysocki]

Summary

This PR adds Mapbox to the Partner Skills section of the README.

What's Included

Mapbox has created four comprehensive Agent Skills for Claude that teach best practices for building maps:

• 🎨 mapbox-cartography - Map design principles, color theory, visual hierarchy, typography
• 🔐 mapbox-token-security - Token management, scope control, URL restrictions, rotation strategies
• 📐 mapbox-style-patterns - Common style patterns and layer configurations
• 🔧 mapbox-integration-patterns - Framework integration patterns for React, Vue, Svelte, Angular

Repository

All skills are available in the Mapbox MCP DevKit Server repository:
https://github.com/mapbox/mcp-devkit-server/tree/main/skills

These skills complement Mapbox's MCP server which provides tools for creating styles, managing tokens, and working with the Mapbox platform.