Improve OpenPGP documentation

• Link to the OpenPGP features of Seahorse, for example the key management feature.
  • Link to general introduction to OpenPGP, for example:
    • http://www.gnupg.org/gph/en/manual.html
  • Mention Enigmail and point to the Thunderbird documentation

“Paperkey reduces the amount of data
to be printed to the bare minimum (secret part of key) and adds
checksums which allow for easier restore of the key when typing it in or
dealing with dirty scans.”

Subtasks

• #6568
• #9407
• #9404 (closed)
• #15174 (closed)

Related issues

• Related to #8999 (closed)
• Related to #9351 (closed)

Original created by @tails on 5316 (Redmine)