Add a "docling/cpu-ubi9" test collection to https://gitlab.com/redhat/rhel-ai/core/testcollections/pipeline

• "collections/docling/cpu-ubi9" with requirements and empty constraints
• ".gitlab-ci.yml" jobs

For starters only add "docling" without any version constraint or additional dependencies. In theory just "docling" should give us the latest version with all necessary dependencies. If some dependencies are missing, then the requirements can be augmented in a subsequent MR.