Add text to speech entity (#1754)

docs/core/entity/tts.md

@@ -0,0 +1,55 @@
+---
+title: Text To Speech Entity
+sidebar_label: Text To Speech
+---
+
+A text-to-speech (TTS) entity enables Home Assistant to speak to you.
+
+A text-to-speech entity is derived from the  [`homeassistant.components.tts.TextToSpeechEntity`](https://github.com/home-assistant/core/blob/dev/homeassistant/components/tts/__init__.py).
+
+## Properties
+
+:::tip
+Properties should always only return information from memory and not do I/O (like network requests).
+:::
+
+| Name | Type | Default | Description
+| ---- | ---- | ------- | -----------
+| supported_languages | list[str] | **Required** | The supported languages of the TTS service.
+| default_language | str | **Required** | The default language of the TTS service.
+| supported_options | list[str] | None | The supported options like voice, emotions of the TTS service.
+| default_options | Mapping[str, Any] | None | The default options of the TTS service.
+
+## Methods
+
+### Get supported voices
+
+This method is used to return a list of supported voices for a language of a TTS service.
+
+```python
+class MyTextToSpeechEntity(TextToSpeechEntity):
+    """Represent a Text To Speech entity."""
+
+    @callback
+    def async_get_supported_voices(self, language: str) -> list[str] | None:
+        """Return a list of supported voices for a language."""
+```
+
+### Get TTS audio
+
+The get tts audio method is used to generate an audio file from a text message using a TTS service.
+
+```python
+class MyTextToSpeechEntity(TextToSpeechEntity):
+    """Represent a Text To Speech entity."""
+
+    def get_tts_audio(
+        self, message: str, language: str, options: dict[str, Any] | None = None
+    ) -> TtsAudioType:
+        """Load tts audio file from the engine."""
+
+    async def async_get_tts_audio(
+        self, message: str, language: str, options: dict[str, Any] | None = None
+    ) -> TtsAudioType:
+        """Load tts audio file from the engine."""
+```