Creates a new Plugin Development Guide

Since Fuel 6.0 supports Pluggable architecture,
a separate Guide is created to describe the following:

* how to install a plugin
* example plugins
* how to create a plugin
* how plugin UI elements should look like

Links to plug-in-related CRs:

* 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/

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

Change-Id: I78668b7bdc0e1b4f09244146a789bb12c921100c
Closes-Bug: 1397889
Merge "Creates a new Plugin Development Guide"