config.json in directory config mode should be settings.json

[simonw]

Another knock-on effect of #992.

datasette/docs/config.rst

Lines 51 to 55 in 4bac9f1

	* ``metadata.json`` - :ref:`metadata` for those databases - ``metadata.yaml`` or ``metadata.yml`` can be used as well
	* ``inspect-data.json`` - the result of running ``datasette inspect`` - any database files listed here will be treated as immutable, so they should not be changed while Datasette is running
	* ``config.json`` - settings that would normally be passed using ``--config`` - here they should be stored as a JSON object of key/value pairs
	* ``templates/`` - a directory containing :ref:`customization_custom_templates`
	* ``plugins/`` - a directory containing plugins, see :ref:`writing_plugins_one_off`

[simonw]

Part of #1105

[simonw]

I'll throw an error if a config.json file is detected.