Add notebooks to indexes

Notebooks currently do not appear in lists of examples and must be accessed through a link in an associated example page. It would be very useful to be able to discover notebook directly from a list of examples for a given module.

Start by locating the code that builds lists of examples for each module and determine how notebook examples are represented in the existing documentation. The work is done when notebooks can be discovered directly from the relevant example indexes, rather than only through links from associated example pages.

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