ci: move docs-stuff out of root .gitlab-ci.yml
Unlike most other cases, we don't put the YAML-file in a ci-folder, because we already have one for the CI-specific docs. So let's just leave the YAML file directly in the docs-folder.
This should fix the problem that any docs-changes that touches the CI-rules needs a full CI run just because of touching the root .gitlab-ci.yml file. This causes needless friction and wastes CI resources.