Includes a basic HOT template guide

The 2 RST files are copied from the heat developer documentation without
any change and are just a starting point.

This patch requires this change in the openstack-doc-tools:
https://review.openstack.org/#/c/108034/

Change-Id: I03511655c86ead880cadfa68b43bb2b90d3f7c06
Implements: blueprint heat-templates
Merge "Includes a basic HOT template guide"