Clarify that BitTorrent verification doesn't replace OpenPGP verification

Originally created by @muri on #11019 (Redmine)

https://tails.boum.org/install/linux/usb/index.en.html states:

Your BitTorrent client verifies your download automatically based on a cryptographic checksum in the Torrent file.

but users should verify the downloaded iso using gpg. we should make clear that these are different kinds of verification.

Feature Branch: web/11019-bittorrent-openpgp

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Edited by muri
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