Add examples for development of Fuel

Adding a new page to Fuel development
documentation including two specific examples -
the addition of Zabbix role, and the addition of
support for Mellanox hardware.

Change-Id: Ib5f7df8cc0c94580bc8c3deff61c6934dd701079
Implements: blueprint fuel-web-docs-dev-env-restructure
Merge "Add examples for development of Fuel"