Revise the `Shared wallet` reference design

[GBKS]

This page has not been updated in a while. Content is slim and the visuals are minimal compared to the Daily spending wallet reference design. There are probably also more opportunities for cross-referencing relevant content now.

This issue is extracted from #717.

[Bosch-0]

@sbddesign you wanted to tackle this yeah?

[rabbitholiness]

If it's ok, I'll pick this one up. I've been working on the Miniscript/multi-key topics and this seems like a natural extension.

[sbddesign]

If it's ok, I'll pick this one up. I've been working on the Miniscript/multi-key topics and this seems like a natural extension.

Go for it @rabbitholiness

[samhithakeluth]

@rabbitholiness Can I volunteer, too? If so, then is there any existing Figma file I can work with?