Design multi-key wallet creation flow

[GBKS]

This is a follow-up to #62. We have the single-key flow nicely documented in the design docs. Let's also wrap up the multi-key wallet creation flow, for which we have some design explorations floating around (here and here).

It would be good to go through the usual steps:

• Discover: write some specs on what needs to be considered and do explorations
• Define: Whittle down explorations into a direction
• Design: Finalize the direction and figure out all the details
• Deliver: Document it all so it can go into implementation