GitLab

Summary:
* Introduce the mini-index at `torchaudio.transforms` page.
* Add "Augmentations" subsection.
* Also updated the overall introduction.

https://output.circle-artifacts.com/output/job/1b65246a-403c-4d2c-b97d-d1b582d8b4e5/artifacts/0/docs/transforms.html

<img width="721" alt="Screen Shot 2022-09-16 at 5 20 08 PM" src="https://user-images.githubusercontent.com/855818/190591795-97c169db-a95b-480a-8d3c-d80072efa045.png">

<img width="755" alt="Screen Shot 2022-09-16 at 5 20 28 PM" src="https://user-images.githubusercontent.com/855818/190591828-03026918-febd-4194-91aa-7d8f704e17cc.png">

Pull Request resolved: https://github.com/pytorch/audio/pull/2683

Reviewed By: carolineechen

Differential Revision: D39574255

Pulled By: mthrok

fbshipit-source-id: a4beed7cacbb5184bad96efa903a3a1123dab627