Grammar sphinx pages don't build with sphinx <= 1.6.7

Fails on pkg job on Debian 9 and Ubuntu 18.04: https://jenkins.aws.isc.org/job/kea-dev/job/pkg/408

The problem seems to be that it needs every .rst page to be part of a table of content.

14:26:36  Warning, treated as error:
14:26:36  /tmp/workspace/kea-dev/pkg/kea-src/kea-1.9.9/doc/sphinx/grammar/grammar-ca-parser.rst:document isn't included in any toctree