Inheritance wallet - Part 3 - Cosigner onboarding & key replacement

[rabbitholiness]

This PR builds on the previous inheritance wallet PRs: Part 1: Use case & wallet creation Part 2: Wallet backup

It adds a page about how Alice is set up as a cosigner for the family savings wallet. It also adds a page about how making changes work, like key replacement or editing timelocks.

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[rabbitholiness]

Opened this PR in draft mode so far, so that people can look at it. There's quite a few things to tweak:

• Image captions & alt-texts
• Consistency checks
• Navigation & reference links
• Potentially additional clickable prototypes

[rabbitholiness]

Opening this up for initial feedback, even if there are two things left to do:

• Write alt-texts for the 66 images in this PR 😊
• Create a visual for the "how it works" section of the "Making changes" page, for better clarity.

[rabbitholiness]

I will open a new PR, as I had technical issues with the existing branch.