Update plugin documentation to fully use pyproject.toml

[Czaki]

🧰 Task

Our plugin creating documentation https://napari.org/stable/plugins/building_a_plugin/first_plugin.html#update-setup-cfg is cuild mainly around setup.cfg when we have migrated our template to pyproject.toml https://github.com/napari/cookiecutter-napari-plugin/blob/main/%7B%7Bcookiecutter.plugin_name%7D%7D/pyproject.toml

We should update our documentation to make all examples using pyproject.toml.
We may think about including also hatch example build backend (as it is suggested backend for pure python packages).