nbtest improvements

[miguelgrinberg]

This PR adds a few improvements to the nbtest script.

• Move the configuration of masks to an external .nbtest.yml config file
  (Fixes https://github.com/elastic/enterprise-search-team/issues/6253)
• Add "set up" and "tear down" notebooks for additional logic necessary to keep the tests running cleanly
  (Fixes #https://github.com/elastic/enterprise-search-team/issues/6252)
• Up to date nbtest documentation in test/nbtest/README.md
  (Fixes #https://github.com/elastic/enterprise-search-team/issues/6068)
• Minor improvements to the makefiles in notebook/search and notebook/document-chunking, so that you can also say make <notebook>
• Moved the cleanup that existed at the bottom of the document-chunking notebook to a tear down notebook, and also added logic to remove the model, which was missing.
• Implemented a setup notebook for notebooks/search that creates book_index for the notebooks that expect this index to exist ahead of time.
• Implemented a teardown notebook for notebooks/search that destroys all the indexes that are created.
• Implemented a specific teardown notebook for notebooks/search/03-ELSER.ipynb that removes index, pipeline and model.
  Fix elastic/enterprise-search-team#6068