Document our vagrant based build setup in Jenkins
_Originally created by @bertagaz on [#12616 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/12616)_
Once we’ve finalized the deployment of the vagrant based build setup in
Jenkins and have polished it so that it’s robust enough, we’ll have to
move part of what is written in the blueprint in a design documentation.
Feature Branch: master
Parent Task: tails/tails#5630
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