Add ⚡️ content to `Private key management` > `Manual backup`

[GBKS]

First step is to review the page for additions and changes.

Ideas:

• Add note about potentially backing up derivation path, fingerprint, etc
• Add note about limits of manual backup - descriptors, Lightning channels, etc are all too complex to write down

[sbddesign]

I can take this one as well. There will be some overlap with what I have been working on in the Onboarding > Backups section.

[danielnordh]

Closed by #559