Expand how PINs work in secure wallets

[GBKS]

Steyn has been investigating exactly how PINs can protect user wallets - see the Slack thread. This came up due to a brief mention of PINs in the savings wallet reference design, without further explanation. He'll create and share a write-up that we can then add to the existing page, or create a new page for.

[steynviljoen67]

Thanks for creating the issue, Christoph. You can assign it to me. 🙏