Update hardware accelerated video processing tutorial (#3050)

Summary:
Par https://github.com/pytorch/audio/issues/3040 and https://github.com/pytorch/audio/issues/3041, it turned out Google Colab now has FFmpeg with GPU decoder/encoder preinstalled, and installing FFmpeg manually corrups the environment.

This commit updates the tutorial by extracting and moving the how-to-install part to installation/build section.

closes https://github.com/pytorch/audio/issues/3041
closes https://github.com/pytorch/audio/issues/3040

Pull Request resolved: https://github.com/pytorch/audio/pull/3050

Reviewed By: nateanl

Differential Revision: D43166054

Pulled By: mthrok

fbshipit-source-id: 32667f292a796344d5fcde86e8231e15ad904e58

docs/source/index.rst

@@ -32,6 +32,7 @@ model implementations and application components.
    build.linux
    build.windows
    build.jetson
+   build.ffmpeg
 
 .. toctree::
    :maxdepth: 1