Add an epiloge on the install instructions about creating a Tails backup

Some issues faced by our users come in a very stressed tone because users lose private keys, electrum wallets, etc. because of the lack of backups.

Maybe we could add a paragraph about backups to the end of the install instructions, with links to https://tails.boum.org/doc/first_steps/persistence/backup/ and some comment about the insufferable levity of USB sticks ?

It may sound obvious to knowledgeable people, but many users don't realise their USB stick may fail somehow or forget to make the backups needed.