[Website]: Restore of Fleet snapshots from one deployment to another

### Type of issue

None

### What documentation page is affected

https://www.elastic.co/docs/reference/fleet/migrate-elastic-agent

### What happened?

According to some issues reported by some user, we should review if the procedure detailed at the doc page:
- Still works when Saved Object Encryption is used (if the user needs to somehow provide the old encryption key prior to restoring)
- If there are caveats post restore

### Additional info

_No response_