Improve user onboarding and initial setup

[jbilcke-hf]

Context

Today using Clapper for the first time can be a bit confusing, because we don't know where to setup API keys etc

I haven't worked a lot on this part since key setup is a bit of "do it once, then forget about it" process, that the user doesn't need to do often - but this is still important!

Here is the current flow:

TODO

We should work on an improved onboarding process. One solution could be to display a popup the first time we try to generate, or when there is an error "You need to setup your API key, click here" etc

[sendn0des]

I will evolve this with better screenshots of the potential advance onboard flow. We need to re-review this but I think it's in a good place that'll work for people @lalalune @jbilcke-hf would love to do that with you guys on a call one last time