By default, the source of the merge is `${CI_COMMIT_TAG}` and the
target must be defined by the `.gitlab-ci.yml` job using this
template.

* docs/GETTING-STARTED.md: new section to document `.git:merge-to`
  template job.

* templates/Git.yaml: new `.git:merge-to` template job to merge a
  reference into a target branch.

Using `commitlint` require the users to include `templates/Git.yaml`
and `extends` the `.git:commitlint` job template.

We keep `templates/Lint/Commitlint.yaml` for backward compatibility.

* templates/Git.yaml: new git related job templates

* templates/Lint/Commitlint.yaml: define `commitlint` with the
  `.git:commitlint` job template for backward compatibility.

* .commitlintrc.yaml: move from JS to YAML.