List of Registries not displayed

## 📚 Documentation Issue

* The **Write Models** page https://detectron2.readthedocs.io/en/latest/tutorials/write-models.html says "A complete list of registries can be found in [API documentation](https://detectron2.readthedocs.io/en/latest/modules/modeling.html#model-registries)." The** list of registries are not displayed at the link https://detectron2.readthedocs.io/en/latest/modules/modeling.html#model-registries. The file detectron2/docs/modules/modeling.rst does have a list at the bottom (which includes detectron2.modeling.META_ARCH_REGISTRY, etc.).

* How should the above documentation/comment/tutorial improve:
Make the list of registries visible at https://detectron2.readthedocs.io/en/latest/modules/modeling.html#model-registries.

Start with detectron2/docs/modules/modeling.rst and compare its registry list with the rendered model-registries section linked from the Write Models page. Check the documentation build or rendered API page to determine why the list is not visible. Done means the complete registry list appears at the model-registries link and the tutorial reference is accurate.

Written by the indexing model from the issue text.