Explain how to activate a new language on the website

We only activate a new language on the website once the core pages are
translated. Those are listed on
https://tails.boum.org/contribute/how/translate/#index3h1.

Then we deactivate languages if they go below 25 % of those core pages.

Feature Branch: bitingbird:contrib/7223-new_language

Original created by @sajolida on 7223 (Redmine)

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