Link to upgrade instructions in Tails Installer when a Tails USB stick is selected

Originally created by @emmapeel on #7904 (Redmine)

When opening the Tails installer, there is a documentation link pointing to the installation assistant.

But this link doesn’t cover all options for the Tails installer, it just talks about upgrading the USB stick with Clone & Install.

So, we should consider providing a link for the upgrade options (Manual upgrade using Tails Installer).

Feature Branch: bugfix/7904-15590-14849-14720-14810-installer-fixes-for-3.9, kurono/installer:bug/7904-change-link-upgrade-instructions

Edited May 15, 2020 by emmapeel

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