Add docs for SpeakerManager

docs/source/main_classes/speaker_manager.md

@@ -0,0 +1,11 @@
+# Speaker Manager API
+
+The {class}`TTS.tts.utils.speakers.SpeakerManager` organize speaker related data and information for 🐸TTS models. It is
+especially useful for multi-speaker models.
+
+
+## Speaker Manager
+```{eval-rst}
+.. automodule:: TTS.tts.utils.speakers
+    :members:
+```