[Tails-l10n] Decide what to do with documentation about very…

xin raised a good point to me: why should documentation about very old
transitions, like /doc/first_steps/persistence/upgrade (October 2013) be
translatable? New translation teams should definitely not translate them.

I created a ticket to discuss this, with the possible solutions, and I'd
like your opinion:

https://labs.riseup.net/code/issues/11538

Let's discuss here and move the conclusion only to the ticket.