[docs] Add torch.package documentation preamble (#59491)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59491
**Summary**
This commit adds a preamble to the `torch.package` documentation page
that explains briefly what `torch.package` is.
**Test Plan**
Continous integration.
<img width="881" alt="Captura de Pantalla 2021-06-04 a la(s) 3 57 01 p m" src="https://user-images.githubusercontent.com/4392003/120872203-d535e000-c552-11eb-841d-b38df19bc992.png">
Test Plan: Imported from OSS
Reviewed By: Lilyjjo
Differential Revision: D29050630
Pulled By: SplitInfinity
fbshipit-source-id: 70a3fd43f076751c6ea83be3ead291686c641158
