Refactor trusted reproducer howto

The main goal of this is to remove the need for a third person (not the RM nor the TR) to send an email including the "TR-bits".

While doing so, I also tried to clean up the doc.

I have removed the need to download images; it might seem uninteresting (most of us needs to keep isos.git updated anyway), and I acknowledge the issue is not to spare 4GiB of traffic nor some minute. But it was making things more complicated (more variables, more complex paths, etc.). So those are side-effects in a general attempt to simplify the process.