Warn user or prevent accidental deletion of the original document

[Extarys]

Is your feature request related to a problem? Please describe.
Deleting the original document breaks the signature of the signed document.

It was unclear to me if I could delete the original file after it was signed (no, don't) or even if I can move the original and/or the signed file.

Describe the solution you'd like
Prevent the deletion of the file
Or (easier for now), clearly state when adding people for signature that the original document should be left untouched.

Describe alternatives you've considered

Additional context
This is a plaster 🩹 while #4165 is implemented 🤞

[vitormattos]

We will need to go ahead with #4165 before start to talk at this issue.

[Extarys]

I know 😄 no worries