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.

将 tensorflow~python-activations 示例中显示的标准 topic/type 页面与 tensorflow~python/tf/keras/activations 中现有的自定义索引进行比较。跟踪 topic/type 页面如何选择其 homepage，并确定如何改为指定某个特定的文档页面。完成标准是：可以为适用的 types 选择自定义索引页面，同时不改变其他页面的标准格式。

由索引模型根据 Issue 内容生成。