Reorganize the contributing docs

Give up on trying to render out contributing docs as plain RST at <https://github.com/okfn/ckan/blob/master/CONTRIBUTING.rst>, they're too long for that. Replace that file with just a link to <http://docs.ckan.org/en/latest/contributing>, and reorganize the contributing docs using proper Sphinx features (tables of contents, etc.)

All the contributing docs go into `docs/contributing/`. `docs/contributing/index.rst` is the root file (included in the main docs tables of contents) that includes a table of contents of all the other files in contributing. For some topics (coding standards, frontend development) there are further subdirectories with their own index files, e.g. `doc/contributing/coding-standards/*.rst`. It's all arrange in a way that works nicely with the global table of contents and navigations links.

This is finished, the only problem is I'm getting these two Sphinx warnings, which I don't understand:

```
/home/seanh/Projects/ckan/ckan/doc/contributing/coding-standards/testing-coding-standards.rst:30: WARNING: Include file u'/home/seanh/Projects/cka
n/ckan/doc/contributing/coding-standards/../ckan/new_tests/logic/action/test_update.py' not found or reading it failed
/home/seanh/Projects/ckan/ckan/doc/contributing/coding-standards/testing-coding-standards.rst:16: WARNING: Include file u'/home/seanh/Projects/cka
n/ckan/doc/contributing/coding-standards/../ckan/new_tests/logic/auth/test_update.py' not found or reading it failed
```

There are no includes on lines 16 and 30 of that file, and the includes of `test_update.py` that do exist in the file seem to be working fine.