concepts: Add GnuPG-replacement document

Add a document to describe the replacement of GnuPG with a GPL-3-free
alternative. Include different options in a comparison and a plan for
the transition.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

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