Merge branch 'docs/add-toc' into 'master'

Start using 'toc' in yaml frontmatter to explicitly disable it

See merge request !644

docs/index.md

+---
+toc: false
+---
+
 # GitLab Runner
 
 [![Build Status](https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/badges/master/build.svg)](https://gitlab.com/gitlab-org/gitlab-ci-multi-runner)