Re-organise top-level template guide structure

The current structure must be baffling for new template authors.

First is a section on Environments, which is actually an advanced
topic. Then is a section on cfn compatible functions which users
should be spurning in favour of HOT intrinisic functions.

The actual docs on how to write a HOT template and the HOT spec
is buried at the end.

This change remedies this state of affairs by moving the
Heat Orchestration Template (HOT) Guide to the start, followed by
the HOT spec.

The Environments section is now after the resource types, and the
Functions section has been renamed CloudFormation Compatible
Functions.

How we document CloudFormation coverage probably needs to be
rethought, but that is beyond the scope of this change.

Change-Id: I2ef2568309427b37555bae3d6b24a1464a7379b1
Merge "Re-organise top-level template guide structure"