Add stub chapter for heat to config-ref

This patch adds the previously generated tables for heat into the
config reference by way of a new chapter.

It is expected that this be merged as a starting point, with structure
to enable further patching.

Closes-Bug: 1284470
Change-Id: I715849f542c4b1c291fdc6a1c669f77cabca18ff
Merge "Add stub chapter for heat to config-ref"