feat(git): new `.git:merge-to` job template to merge a reference

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.