Move main_classes to a separate folder

docs/source/main_classes/gan.md

@@ -0,0 +1,12 @@
+# GAN API
+
+The {class}`TTS.vocoder.models.gan.GAN` provides an easy way to implementing new GAN based models. You just need
+to define the model architectures for the generator and the discriminator networks and give them to the `GAN` class
+to do its ✨️.
+
+
+## GAN
+```{eval-rst}
+.. autoclass:: TTS.vocoder.models.gan.GAN
+    :members:
+```