Update theming docs for 2.0

I think these pretty much cover the 1.x theming and need to be completely rewritten: <http://docs.ckan.org/en/latest/theming.html>

Some things they need to cover:

- [x] Different ways of customising CKAN's appearance. There are ini file settings and `/ckan-admin`, then there are custom public and template dirs you can specify in your ini file, then there are extensions with their own public files and templates
- [x] Jinja block-based inheritance
- [x] `{% ckan_extends %}` and any other important custom tags
- [x] What theme developers need to know about fanstatic? e.g. the template code for pulling in fanstatic resources, defining your own fanstatic resources, minifying files
- [x] What do theme developers need to know about less?

There are a couple of pages, not linked to from the toc or any other page, that should be incorporated into this and deleted:

https://github.com/okfn/ckan/blob/master/doc/extension-templating.rst

https://github.com/okfn/ckan/blob/master/doc/templating.rst

https://github.com/okfn/ckan/blob/master/doc/template-tutorial.rst

https://github.com/okfn/ckan/blob/master/doc/template-blocks.rst

This may also be relevant: https://github.com/okfn/ckan/blob/master/doc/resources.rst