doc: Add Migration Guide from Mesa, Mesa-geo to Mesa-LLM

## Description

Add a comprehensive migration guide for users moving from **Mesa** or **Mesa-Geo** to **Mesa-LLM**.

The guide should provide a practical, step-by-step migration path and explain how existing Mesa and Mesa-Geo models can be adapted to work with LLM-powered agents in Mesa-LLM.

### Scope

The migration guide should cover:

* Key differences between **Mesa, Mesa-Geo, and Mesa-LLM**
* Mapping common Mesa concepts and workflows to Mesa-LLM
* Migrating existing Mesa models to Mesa-LLM
* Migrating **Mesa-Geo** models and geospatial workflows to Mesa-LLM
* Adapting agents to use LLM-powered reasoning and memory
* Configuring LLM providers and models
* Common migration issues and troubleshooting
* Links to relevant Mesa-LLM tutorials and documentation

### Expected Outcome

A clear, beginner-friendly migration guide that helps existing **Mesa and Mesa-Geo users** understand what changes are required to adopt Mesa-LLM and migrate their existing models and workflows.

Start by reviewing the existing Mesa, Mesa-Geo, and Mesa-LLM tutorials and documentation. Compare their concepts, model workflows, geospatial workflows, agent configuration, and provider setup, then document a step-by-step migration path with troubleshooting and links. Done means Mesa and Mesa-Geo users can understand the required changes and adapt their workflows to Mesa-LLM.

Written by the indexing model from the issue text.