[#973] Make sure we always talk about datastore resource and not just resources, extend introduction to include motivation for the datastore
[#973] Improve references in data viewer docs
[#973] Restructured Datastore description. Introduction is much simpler and shorter.
Merge branch 'master' into 973-datastore-docs-improvements

Conflicts:
	doc/data-viewer.rst
[#973] Fix merge
Merge branch 'master' of github.com:okfn/ckan into 973-datastore-docs-improvements
Merge branch '973-datastore-docs-improvements' of github.com:okfn/ckan into 973-datastore-docs-improvements
[#973] Fix a broken Sphinx reference
Merge branch '973-datastore-docs-improvements'
Fix broken IResourcePreview docstring

Not spell-checked, not following CKAN's docstring guidelines, contained
Sphinx errors. I also rewrote it for clarity.
Fix a Sphinx warning