choose a markdown style and avoid warnings in codacy

[nicolaslg]

Codacy warns about coding-style non-conformity in our markdown files.

remark-lint seems to be used with some preset applied, that we can configure per our wishes. While I have no preference whatsoever in a particular coding-style for markdown files, I want to get rid of those warnings.

I suggest we choose a style (why not the one currently enforced by codacy ?) and adhere to it.