Add a global index.md for docs
This is needed in order to have GitLab Runner listed on docs.gitlab.com.
https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/11
- The project's README.md points to the docs portal instead of the source files
- Every directory now has a readme or index file
Closes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1798