Clean images directories

[celine-sncf]

There is a general problem on the images of the website (excluding the ones in /static dir, obviously): only the ones in the same dir than the md file and linked using markdown syntax are rendered in /fr (french version of the website).

Currently, the solution would probably be to:

• migrate all images (and links) into /static dir 😓.
• probably add a CI check to forbid images into /content dir.

The static dir could also be cleaned / reorganized.

[Pivouane]

Most of the work related to markdowns including relative paths (mostly logos) done but i suggest not implementing a CI rule that forbids images in /content.

This goes against good habits and hugo bundling principles, I'll link an explanatory part of CONTRIBUTING.md here once written to make sure of what images should go in /content and what images should go in /static.

I also suggest making a bigger issue about images-related problems, as it seems some articles refer to absent images but it clearly is a bigger task than just cleaning the static folder.

[bougue-pe]

Re-opening it as some sub-issues are not listed.