Improve example notebooks

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Document the notebooks in the etc directory.

• Add examples of the %inject magic in the https://github.com/jupyter-incubator/contentmanagement/blob/master/etc/notebooks/test_cookbooks.ipynb notebook
• Add examples of invoking notebook functions as APIs in the same notebook
• Add markdown describing what is going on in both cases
• Introduce search dialog in the same notebook and the "insert ..." links it provides
• Add a small footnote about the ToC and bundler options (refer to other notebook)
• Rename that notebook to something better than "test_"

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/cc @gbishop

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Working on this in PR #23.