Move main_classes to a separate folder

docs/source/main_classes/model_api.md

@@ -0,0 +1,24 @@
+# Model API
+Model API provides you a set of functions that easily make your model compatible with the `Trainer`,
+`Synthesizer` and `ModelZoo`.
+
+## Base TTS Model
+
+```{eval-rst}
+.. autoclass:: TTS.model.BaseModel
+    :members:
+```
+
+## Base `tts` Model
+
+```{eval-rst}
+.. autoclass:: TTS.tts.models.base_tts.BaseTTS
+    :members:
+```
+
+## Base `vocoder` Model
+
+```{eval-rst}
+.. autoclass:: TTS.tts.models.base_vocoder.BaseVocoder`
+    :members:
+```