Add nightly doc update (#3496)

Summary:
Add scheduled doc update job so that docs are updated at least once a day.

Pull Request resolved: https://github.com/pytorch/audio/pull/3496

Differential Revision: D47795577

Pulled By: mthrok

fbshipit-source-id: aba5376ec51f07560014d250a16fef8b8a11b43e

.github/workflows/build_docs.yml

@@ -10,6 +10,8 @@ on:
       - v[0-9]+.[0-9]+.[0-9]
       - v[0-9]+.[0-9]+.[0-9]+-rc[0-9]+
   workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * *'
 
 jobs:
   build: