[DOC] - Add FAQ

[trallard]

Creating a new issue to make review - and follow through easier (Also I will be working on this 😉)

---

I think this is a good direction because we are frequently asked the same questions.

Several things are definitely worth adding in my opinion:

• why we do not add your favorite package :)
• issues we have with arm images and x86 also, related to our build process
• how to give root permissions to the user
• who jovyan is :)

I mean, these are the questions I saw quite frequently and it's not obvious how to answer them.
And, of course, it's completely up to you which questions you would like to answer in this PR.

Originally posted by @mathbunnyru in #1589 (comment)

[mathbunnyru]

@consideRatio @maresb @romainx @trallard feel free to add new frequently asked question to the list.