Mention I2P in "About" page

Our current “About” page does not mention I2P. It might be ok to not mention it on the homepage but it should be briefly introduces in the “Online anonymity” of the “About” page, as an alternative to Tor. Not everything goes through Tor, I2P does not.

Feature Branch: doc/6533-about_i2p

Original created by @sajolida on 6533 (Redmine)

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