Start User guide

[benoitblanc]

Hello,

As discussed in previous issues, this PR aims to start a user guide.

For now, it presents an overview of the application with basic features.

I was thinking about creating documentation in each repository and fetch content by editing Makefile but I don't know how to manage images links (in hugo, it needs /images/image.png and in other repo, the first / causes an issue so there is no displayed image in repository). While looking for a solution, we can create the content directly in this repo and switch back to the different repositories to fetch content from all the repositories later.

Thanks

[pka]

Thanks Benoît for this great addition. There are probably already a few existing user guides, like this one in German.