add a section about onboarding

[r10s]

we should describe briefly what is going on, for deeper reasonings etc. we'll probably link to a blog post that will come with 1.46 as well

the onboarding section is then probably linked from the welcome pages; in general, however, it makes sense to target this issue comparable late, currently too many things are under construction and not 100% settled.

[hpk42]

If i am not mistaken, then #838 resolved this by introcuing the "what is chatmail" option to be shown from "explore other options" in the create-profile screen.