Mention Enigmail in documentation

We currently do not have a documentation about using OpenPGP in Tails. See #5316.
Besides mentioning Seahorse, we should also mention Enigmail and the OpenPGP Applet (which has a proper documentation).

Parent Task: #5316

Related issues

Related to #6568
Related to #9407

Original created by @u on 15174 (Redmine)

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