Rework doc/source/index.rst as landing point for all heat docs

[Steve Baker]

With Big Tent, much of heat's documentation will end up being in http://docs.openstack.org/developer/heat/. This means that the landing page index.rst needs to be reorganised to serve the needs of different audiences:

- heat users
- heat operators
- heat developers

Once this is complete, https://wiki.openstack.org/wiki/Heat content should be replaced with a link to http://docs.openstack.org/developer/heat/index.html

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.openstack.org/216486

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/216486
Committed: https://git.openstack.org/cgit/openstack/heat/commit/?id=fc5834bb7fbd89d121652761c19a53088454d17a
Submitter: Jenkins
Branch: master

commit fc5834bb7fbd89d121652761c19a53088454d17a
Author: Steve Baker <email address hidden>
Date: Tue Aug 25 13:53:57 2015 +1200

    Use the official mission statement in index.rst

    Also, since this is not primarily developer documentation any more,
    remove the word 'developer' from the page title.

    Change-Id: Ia3fb3aae150f6439bc8c8b0eac71b634dca3d445
    Related-Bug: 1488292