[ci] Pipeline refactoring

• Scheduled pipeline for wheel build (will setup a weekly build)
• ci::web only pipeline: just add this label to the MR to limit CI to Jupyter notebook runs and web site creation
• Restructured CI job files a bit:
  • scripts/ci/pipelines/*.yml: Different pipeline configs: regular, web only and scheduled
• Changed ci skip-label to ci::skip

There will be more labels for reduced pipelines in the future. I can imagine e.g. ci::[platform] only.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?