Help for the Shiny app

[ghost]

It would be helpful to add a "Help" tab (before the about one) that allows us to read the documentation of the package (a least), and a specific guide for the shiny version (at best).

[nfrerebeau]

Displaying the package documentation in the application is not relevant. A dedicated help for using the app would be more appropriate, but I'm not sure we need to add a tab for that.

A dynamic help system may be a better way: https://shiny.rstudio.com/articles/js-introjs.html

I'll keep the issue open until we figure out the solution.