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.