Add docs for media player group (#14928)

* Add media group docs

* Fix configuration example

* Bump ha_release

Co-authored-by: Franck Nijhof <frenck@frenck.nl>

source/_integrations/media_player.group.markdown

@@ -0,0 +1,49 @@
+---
+title: "Media Player Group"
+description: "Instructions for how to setup Media player groups within Home Assistant."
+ha_category:
+  - Media Player
+ha_release: "2021.7"
+ha_iot_class: Local Push
+ha_quality_scale: internal
+ha_domain: group
+---
+
+The group media player platform lets you combine multiple media players into one entity. All child media players of a media player group can still be used as usual, but controlling the state of the grouped media player will forward the command to each child media player.
+
+To enable this platform in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+media_player:
+  - platform: group
+    entities:
+      - media_player.kitchen_tv
+      - media_player.livivng_room_tv
+```
+
+{% configuration %}
+  name:
+    description: The name of the media player group. Defaults to "Media Group".
+    required: false
+    type: string
+  entities:
+    description: A list of entities to be included in the media player group.
+    required: true
+    type: [string, list]
+{% endconfiguration %}
+
+## Script Example
+
+Here's an example of a script using the above media player group.
+
+```yaml
+script:
+  tts:
+    alias: Run TTS
+    sequence:
+      service: tts.google_translate_say
+      entity_id: media_player.media_group
+      data:
+        message: 'The cake is a lie.'
+```