#255 update api docs

Docs built here: <https://ckan.readthedocs.org/en/255-update-api-docs/#the-ckan-api>

- [x] Move `apiv3.rst` -> `api.rst`, the Action API / API v3 becomes the default API
- [x] Edit and rearrange `apiv3.rst`
- [x] Remove the API quickstart tutorial as superfluous.
- [x] Move the Model API docs (including API versions and HTTP Status codes) into `legacy-api.rst`.
- [x] Move the Search API and Util API docs into Legacy API.
- [x] Update `index.rst`
- [x] Moved Storage API docs out of FileStore and put them next to the other API docs (also renamed Storage API -> FileStore API, added link to ckanclient code that shows how to use it, and edited the FileStore and FileStore API docs a little)
- [x] Removed outdated ckanclient and other language API clients from the docs, moved these to [a wiki page](https://github.com/okfn/ckan/wiki/CKAN-API-Clients)
- [x] Update the _Importing Data_ section, which currently mentions ckanclient
- [x] Break up action api ref into multiple pages
- [x] Put autodoc function references into source-code order (was alphabetical order)
- [x] Edit DataStore API docs
- [x] Need to mention version numbers for action api