broken link in builtin_datasets documentation

## 📚 Documentation Issue
Hey guys, I've been studying docs and noticed some links can be fixed.

https://detectron2.readthedocs.io/en/latest/tutorials/builtin_datasets.html has a broken link to
https://github.com/facebookresearch/detectron2/blob/main/MODEL_ZOO.md

* How should the above documentation/comment/tutorial improve:
Move datasets/README.md to docs/tutorials/builtin_datasets.md and fix a link to MODEL_ZOO.md.
Additionally, in https://detectron2.readthedocs.io/en/latest/tutorials/getting_started.html
change a link from [datasets/README.md](https://github.com/facebookresearch/detectron2/blob/main/datasets/README.md) to https://detectron2.readthedocs.io/en/latest/tutorials/builtin_datasets.html

Start with datasets/README.md and the existing docs/tutorials/builtin_datasets.md, then inspect the MODEL_ZOO.md link and the dataset link in the getting_started documentation. Done means the dataset documentation is in the requested location, its MODEL_ZOO.md link works, and getting_started points to the rendered builtin_datasets page.

Written by the indexing model from the issue text.