[#618] Rename contributing -> CONTRIBUTING in docs

The source file in the root of this git repo is still called CONTRIBUTING.rst,
but the symlink to it in docs/ is now lower-case contributing.rst. This
means the docs.ckan.org URL is /contributing.html not
/CONTRIBUTING.html.
[#618] Add contributing docs to docs index
[#618] Fix a broken link in CONTRIBUTING.rst
[#618] Move Coding Standards link to top of CONTRIBUTING.rst

Make it more prominent.