Modify documentation link in tails-installer UI to new installation assistant page

Once #11576 (closed) has been done, tails-installer should point to the new page of the installation assistant. That is because people who run tails-installer won’t need the complete installation instructions, but only the second part of each installation, that is the creation of the “final Tails stick”.

Related issues

Blocked by #11576 (closed)

Original created by @u on 13176 (Redmine)

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