Create a Tutorial for Constructing Plugin Projects

This should show:

1. Creating a plugin project with an MCP server (can refer to basic tutorial from #33)
2. Adding an extensions.py file that utilizes the ExtensionManifest and MCPServerRegistration classes
3. Adding a pyproject.toml file that sets the extensions.py file as an entry point for the package
4. Creating a venv and pip install -e . on the project
5. Running mada-tools available-servers to see that the server we created is now available to use
6. [Optional] Showing how documentation can be extended with mada-tools export-docs (refer to the CZ repo for best way to document this)
7. [Optional] Showing how testing infrastructure is packaged and can be used with from mada_tools.testing import ... (refer to the CZ repo for the best way to document this)

Start with the basic tutorial in issue #33, then review the referenced CZ repository for documentation and testing guidance. Create a tutorial covering extensions.py, pyproject.toml, virtualenv setup, editable installation, and mada-tools available-servers; optionally include export-docs and packaged testing usage. Done means a newcomer can follow the steps to create and discover the example plugin.