.gitlab-ci.yml: Stop depending on specific version of sphinx

This branch fixes #312 by using a hidden toctree to include
the buildstream package and reducing the amount of allowed :orphan:
pages.

.gitlab-ci.yml

@@ -141,7 +141,7 @@ tests-unix:
 docs:
   stage: test
   script:
-  - pip3 install sphinx==1.7.1
+  - pip3 install sphinx
   - pip3 install sphinx-click
   - pip3 install sphinx_rtd_theme
   - cd dist && ./unpack.sh && cd buildstream