Add a terminology article about Fuel plug-ins

Since Fuel 6.0 introduces a new feature, plug-ins,
let's write up an article with plug-in definition,
types and links to other documents.

Links to plug-in-related CRs:

* Separate all-in-one Plug-in Development Guide: https://review.openstack.org/#/c/138013/

* Certification issue (to be posted to Plug-in DevGuide): https://review.openstack.org/#/c/140421/

* User Guide instructions for installing plug-ins: https://review.openstack.org/#/c/130506/

Change-Id: I4e7154353cad9c719afdb4155b53439fa4326374
Closes-Bug: 1397889
(cherry picked from commit e1214de0d162ff01fc2695b4bd264a32a1cd4095)