New model documentation is outdated

[davidorme]

Describe the bug

With the introduction of #388 and #548, our existing documentation on creating new model is outdated:

1. The variable class attributes are incomplete:

virtual_ecosystem/docs/source/development/design/defining_new_models.md

Lines 172 to 187 in c0a84ff

	The {attr}`~virtual_ecosystem.core.base_model.BaseModel.vars_required_for_init` attribute
	: This is a tuple that sets which variables must be present in the data used to create a
	new instance of the model. Each entry should provide a variable name and then another
	tuple that sets any required axes for the variable. For example:
	```python
	() # no required variables
	(('temperature', ()),) # temperature must be present, no core axes
	(('temperature', ('spatial',)),) # temperature must be present and on the spatial axis
	```
	The {attr}`~virtual_ecosystem.core.base_model.BaseModel.vars_updated` attribute : This
	is a tuple that provides information about which data object variables are updated by
	this model. Entries should simply be variable names. The information contained here is
	used to determine which variables to include in the continuous output. So, it is
	important to ensure that this information is up to date.

2. The docs refer to the old manual dependency setting rather than the new system based on the order of variable usage:

virtual_ecosystem/docs/source/development/design/defining_new_models.md

Lines 364 to 402 in c0a84ff

	#### Model dependencies
	Your model may depend on a particular execution order for other models. For example, the
	`freshwater` model might rely on data set up by the `hydrology` model, and so the
	`hydrology` model needs to be initialised and updated before the `freshwater` model.
	This is controlled using model configuration: although these dependencies may be
	strong, it is more flexible to set them up through the configuration process than by
	hard coding dependencies into the model objects themselves.
	Your JSON Schema document therefore needs to include the following at the root level, so
	that the model configuration includes a `[freshwater.depends]` section:
	```json
	"depends": {
	"type": "object",
	"default": {},
	"properties": {
	"init": {
	"type": "array",
	"default": ["hydrology"],
	"items": {
	"type": "string"
	}
	},
	"update": {
	"type": "array",
	"default": ["hydrology"],
	"items": {
	"type": "string"
	}
	}
	}
	}
	```
	Note that this schema provides **default dependencies**, which set which models should
	run before your model. There is no guarantee that users will necessarily include all of
	these models in their configuration and the dependencies can always be overridden by
	users. Configurations that do this may well not work, but that is for users to tackle.

[davidorme]

This should also handle the development of the static model system #373