When the extension is installed, give a short walk-through

[ggrossetie]

It would be great if we could give a short walk-through of the extension to help users get started.
I think we should put emphasis on how to enable this extension on local files and provide a few use cases to spark interest.

We can use the runtime.onInstalled event listener to start the walk-through: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onInstalled

We will need to check the value of OnInstalledReason as we don't ant to show the walkthrough when the extension or the browser is updated: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/OnInstalledReason

An original idea from @marieflorescontact