Custom topic/type pages

All topic/type pages have the same format ([example](https://devdocs.io/tensorflow~python-activations/)): A heading with the doc name and topic name, and a list of all the pages in the topic.

However, some docs have a custom index page for many types already ([example](https://devdocs.io/tensorflow~python/tf/keras/activations)). There should be a way to specify a specific doc page as the homepage for a type.

Compare the standard topic/type page shown in the tensorflow~python-activations example with the existing custom index at tensorflow~python/tf/keras/activations. Trace how topic/type pages choose their homepage and determine how a specific documentation page could be designated instead. Done means custom index pages can be selected for applicable types without changing the standard format for others.

Written by the indexing model from the issue text.