Remove or improve 'Domain Model' docs page

[seanh]

​http://docs.ckan.org/en/ckan-1.8/domain-model.html

Currently not very useful. Also these pages:

​http://docs.ckan.org/en/latest/domain-model-dataset.html

​http://docs.ckan.org/en/latest/domain-model-resource.html

​http://docs.ckan.org/en/latest/domain-model-task-status.html

This just seems very incomplete to me (surely there is more to the domain model the dataset, resource and task status?) and very likely to not get updated as changes are mode to the code. Would anyone seriously refer to this rather than just looking at the source code?

If we want to have something along these lines in the docs, I suggest adding docstrings to packages, modules, classes and methods and then using autodoc to pull them into sphinx.