Added creative commons license to documentation

[peterhcharlton]

Description

This PR aims to add a creative commons license to documentation (as discussed in #769 ).

Proposed Changes

I changed the conf.py file by appending some text to the 'author' field. The text states the license for the documentation.

Further Details

The current commit uses the CC BY 4.0 license, which is the license commonly used in open access publications. The license does require the user to attribute the original source. It allows the authors to retain the copyright.

An alternative, more permissive license is the CC0 license, which places work "as completely as possible in the public domain". This would remove the need for users to attribute the original documentation, and would also mean that the authors opt out of copyright.

The advice in the 'Before licensing' section of this webpage is helpful. Briefly, it's worth noting that "only the copyright holder or someone with express permission from the copyright holder can apply a CC license or CC0 to a copyrighted work."

Checklist

I've tried this on a template sphinx documentation build and it seemed to build ok there. I haven't incorporated it into a build on the full Neurokit documentation.

• I have read the CONTRIBUTING file.
• My PR is targetted at the dev branch (and not towards the master branch).
• I ran the CODE CHECKS on the files I added or modified and fixed the errors.
• I have added the newly added features to News.rst (if applicable)