Gather & analyze data about the Stretch cycle: documentation

That’s one part of #12615. It must be done before the Tails summit starts so we can decide something there. I’ve started this work on the blueprint, see the “XXX” I left in there for you. Assigning to sajolida but I would welcome input from spriver and cbrownstein too :)

Blueprint: https://tails.boum.org/blueprint/Debian_Stretch/#rolling-stretch-analysis

Parent Task: #12615

Original created by @intrigeri on 12691 (Redmine)