Document how we disable Torbutton's update checking

We have a couple of workarounds and specific configurations for making Torbutton not check for updates. This is getting increasingly complicated and now warrants a section in our design document.

Commits of interest in Tails’ Git:

3241197
9267a11 (works around Torbutton commit b9fb03cd)

Related issues

Related to #8469 (closed)

Original created by @anonym on 7284 (Redmine)

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