[docs] Add table of contents to torch.package docs (#59842)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/59842
Test Plan:
Continuous integration.
<img width="544" alt="Captura de Pantalla 2021-06-10 a la(s) 5 13 07 p m" src="https://user-images.githubusercontent.com/4392003/121612390-2ccec280-ca0f-11eb-87ad-fef632ba05ca.png">
Reviewed By: Lilyjjo
Differential Revision: D29050627
Pulled By: SplitInfinity
fbshipit-source-id: 76c25ed4002cbaf072036e2e14e7857c15077df7
