Fix document for MelScale and InverseMelScale (#2967)

Summary:
`InverseMelScale` is missing from the nightly documentation webpage. `MelScale` is better in Feature Extractions section. This PR moves both documents into Feature Extractions section.

Pull Request resolved: https://github.com/pytorch/audio/pull/2967

Reviewed By: mthrok

Differential Revision: D42387886

Pulled By: nateanl

fbshipit-source-id: cdac020887817ea2530bfb26e8ed414ae4761420

docs/source/transforms.rst

@@ -83,7 +83,6 @@ Utility
     :nosignatures:
 
     AmplitudeToDB
-    MelScale
     MuLawEncoding
     MuLawDecoding
     Resample
@@ -100,6 +99,8 @@ Feature Extractions
 
     Spectrogram
     InverseSpectrogram
+    MelScale
+    InverseMelScale
     MelSpectrogram
     GriffinLim
     MFCC