Document Nyx (arm)

We’ve been shipping `arm` in Tails since 2011, and it’s not mentionned anywhere in our doc. That should be fixed.

It should at least be listed in /doc/about/features.
We should also investigate which additional features it bring to advanced users (it runs in a terminal) and explain them in the Advanced Topics sections.

Original created by @sajolida on 8915 (Redmine)

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