Document in Mac install instructions that Apple M1 chip isn't supported

We should document in the beginning of our Mac install instructions that Tails doesn't support the Apple M1 chip.

This is already documented on our system requirements page and in our FAQ. However, @help-desk has been receiving an increasing amount of reports from Mac users complaining that they can't boot their Tails USB sticks. When asked, the users reply that they have Macs with Apple M1 chips.