Mention Seahorse in documentation

At the moment, Seahorse doesn’t have a page in the documentation. We have received reports from users who were thinking that the gpgApplet was the recommended key manager in Tails, which is not the case.

This task implies:

Linking upstream documentation.
Identifying what is specific to Tails regarding Seahorse.
Create a page and linking it from the Encryption & Privacy section, and at least gpgApplet documentation.

Feature Branch: bitingbird:doc/6568-seahorse

Parent Task: #5316

Related issues

Related to #9404 (closed)
Related to #9407
Related to #15174 (closed)

Original created by @sajolida on 6568 (Redmine)

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