These three specifications have been fully implemented:
* Use Kubernetes for Build Resources
* Multiple Ansible versions
* Web Dashboard Log Handling
Remove them from the index since they are not intended as long-term
feature documentation, and the features they implemented were
described in proper documentation as a part of that work.
Also update the index page with a couple of additional sentences
about how spec completion is handled (through removal of the
implemented spec documents) and why we consider it reasonable to do
it this way.
Change-Id: I493d995a329a087341986457bec4b328f1e8b7ea