Change docs files from reStructuredText to Markdown

[berinhard]

All the files under docs/source/ are .rst files, which is the default format used by Sphinx. Although this is fine, it can be little bit annoying to work with this format when compared to Markdown for example.

Expected behavior

After #44, it would be nice to have a better integrity when it comes to file formats in the project. So the expected output from this issue is to have all the .rst files in our docs in .md. To do so, we have to enable Sphinx to handle Markdown files. All the info you need for this is in this session of Sphinx's docs.

[SmileyChris]

I'd argue against bike shed re-painting changes like this. Sphinx by default expects rst. Just live with that default, like you do the choices that black makes for you.

[berinhard]

Thanks for your comment @SmileyChris, but we still want to have a single file format for our docs files, even though it differs from Sphinx's default. The reason is that, by our experience, we notice people are more familiar with Markdown format when compared to reStructuredText.