GitLab

Summary:
* Merge backend doc into torchaudio toplevel doc
* Update backend, dispatcher, installation doc

Pull Request resolved: https://github.com/pytorch/audio/pull/3555

Reviewed By: huangruizhe

Differential Revision: D48326812

Pulled By: mthrok

fbshipit-source-id: cc0d7326eacfebd341323b5d613ca1777255748b