Complete the rst style guide

[satyaog]

Something like :

• https://devguide.python.org/documentation/style-guide/
• https://documentation-style-guide-sphinx.readthedocs.io/en/latest/style-guide.html#
• https://www.john-cd.com/cheatsheets/Markup_and_Documentation/reStructuredText/

should be referenced or explicitly stated in https://github.com/mila-iqia/mila-docs/blob/master/CONTRIBUTING.md .

Then apply the style guide to the whole of the project