text: Title: CI: check docs for newly added source code files
  Body: - [ ] Add CI check that documentation index pages on docs.cleanlab.ai include new source code files which have been added in a new commit. Otherwise somebody may push commit with new source code files, but the documentation for them will never appear on docs.cleanlab.ai.
  
  Ideally we also need to edit less docs/ files and index files to make documentation for new source code files appear in docs.cleanlab.ai. Solutions to minimize number of files that need to be touched are welcomed!
  
  - [ ] Add reminder to CONTRIBUTING.md that lists steps needed to ensure new source code files will have their documentation appear in docs.cleanlab.ai
  
  Currently new source code files must be listed in various places like:
  - The appropriate index files in here: https://github.com/cleanlab/cleanlab/tree/master/docs/source 
  - A module docs page in: https://github.com/cleanlab/cleanlab/tree/master/docs/source/cleanlab
  - The __init__ file: https://github.com/cleanlab/cleanlab/blob/master/cleanlab/__init__.py