Describes Pluggable architecture feature

Since Fuel 6.0 has a brand new feature,
we have the following tasks:

* create instructions on how to install a plugin (User Guide)

Co-Authored-By: Evgeniy L <eli@mirantis.com>

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/

* Updates on Terminology articles: https://review.openstack.org/#/c/140350/

Change-Id: Id44d195c8a70e1b3a943ad7e15dff0753ad8499b
Implements: blueprint cinder-neutron-plugins-in-fuel
Closes-Bug: 1397889