Users get confused by the particularities of the Tails signing key

Originally created by @emmapeel on #11785 (Redmine)

We receive many support requests about users stuck on the verification process
because several applications cannot cope with the fact that we have an expired subkey or sign with the subkey, etc. Examples are:

• Seahorse #11622 (closed)
  Currently can not import the Tails signing key because of this particularity.
  This would be the default graphic way of doing it on Tails.
• Kleopatra
  Recommended on our website for Windows users, has problems to verify the image.

Explaining the particularities of our signing key on some part of the website would be good, only I don’t know where to place it.

Options are:

- Known Issues page
- Download and verify using OpenPGP

Related issues

• Related to #13184 (closed)
• Related to #14977 (closed)
• Related to #15710 (closed)