Explanation how to verify image misses step to import key

After downloading the iso on this page: https://tails.boum.org/install/debian/usb-download/index.en.html
one option to verify the image is to use the command line.
The user should download the public key and the signature.
But the description starts with using the signature without importing the public key which would cause an error message.
This line should be add to 2) Execute:
gpg --import tails-signing.key

Related issues

Is duplicate of #13634

Original created by @sascha.markus@gmail.com on 15448 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information