GitLab

Summary:
1. Reorder Audio display so that audios are playable from browser in doc
2. Add link to function documentations

https://470342-90321822-gh.circle-artifacts.com/0/docs/tutorials/audio_data_augmentation_tutorial.html

Pull Request resolved: https://github.com/pytorch/audio/pull/2082

Reviewed By: carolineechen

Differential Revision: D33227725

Pulled By: mthrok

fbshipit-source-id: c7ee360b6f9b84c8e0a9b72193b98487d03b57ab

* Refactor tutorial organization

* Merge tutorial subdirectoris under to examples/gallery/tutorials
* Do not use index.rst generated by Sphinx-gallery
* Instead use flat structure so that all the tutorials are listed in left menu
* Use `_assets` dir for artifacts of tutorials