Please review monkeysign documentation for Tails, and merge in interesting changes

Hello:

We ship monkeysign with Tails and I am preparing a documentation page to sign keys.

It would be great if you can review it. You can see it here by now:

https://git-tails.immerda.ch/emmapeel/tails/tree/wiki/src/doc/advanced_topics/monkeysign.mdwn?h=docs/11240-monkeysign