Update writing plugins docs to cover pyproject.toml not setup.py

https://github.com/simonw/datasette/blob/b3721eaf507c6d8919b6e72779c1263f297d8c88/docs/writing_plugins.rst#L64-L75

Related:
- #2553 

	Starting an installable plugin using cookiecutter
	-------------------------------------------------

	Plugins that can be installed should be written as Python packages using a ``setup.py`` file.

	The quickest way to start writing one an installable plugin is to use the `datasette-plugin <https://github.com/simonw/datasette-plugin>`__ cookiecutter template. This creates a new plugin structure for you complete with an example test and GitHub Actions workflows for testing and publishing your plugin.

	`Install cookiecutter <https://cookiecutter.readthedocs.io/en/stable/installation.html>`__ and then run this command to start building a plugin using the template::

	cookiecutter gh:simonw/datasette-plugin

	Read `a cookiecutter template for writing Datasette plugins <https://simonwillison.net/2020/Jun/20/cookiecutter-plugins/>`__ for more information about this template.

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