Add an onboarding page

[dannycolin]

Summary

Create an onboarding page

Motivation

An onboarding page will help new users to have a quick overview of what our addon offers.

Additional context

It's still undecided if we're going to use a bundled page or an external one. One downside of the bundled page is that there's an issue that close it if the user check the "allow in private mode" on install.

We also need to think about getting some assets/graphics for the page and a way to organize the locales. For the latter, I (@dannycolin) don't think our current locale files are the best place to put the translations.

[WesleyBranton]

To be honest, I didn't even think about the locale issue. I agree with you about not putting that in the regular locale files, since that would clog things up.

Perhaps having a separate page for each locale is a good way to go?

[dannycolin]

Perhaps having a separate page for each locale is a good way to go?

It won't scale well if we need to change the html/css in the page.