Added 'availability_template' to Template Cover platform (#10314)

* Added 'availability_template' to all Template Cover platform

* Fixed Syntax and default

* Apply suggestions from code review

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Removed optional values from example configuration

source/_components/cover.template.markdown

@@ -19,6 +19,7 @@ To enable Template Covers in your installation,
 add the following to your `configuration.yaml` file:
 
 {% raw %}
+
 ```yaml
 # Example configuration.yaml entry
 cover:
@@ -34,6 +35,7 @@ cover:
         stop_cover:
           service: script.stop_garage_door
 ```
+
 {% endraw %}
 
 {% configuration %}
@@ -62,6 +64,11 @@ cover:
         description: Defines a template to specify which icon to use.
         required: false
         type: template
+      availability_template:
+        description: Defines a template to get the `available` state of the component. If the template returns `true`, the device is `available`. If the template returns any other value, the device will be `unavailable`. If `availability_template` is not configured, the component will always be `available`.
+        required: false
+        type: template
+        default: true
       device_class:
         description: Sets the [class of the device](/components/cover/), changing the device state and icon that is displayed on the frontend.
         required: false
@@ -136,6 +143,7 @@ This example converts a garage door with a controllable switch and position
 sensor into a cover.
 
 {% raw %}
+
 ```yaml
 cover:
   - platform: template
@@ -162,6 +170,7 @@ cover:
             mdi:garage
           {% endif %}
 ```
+
 {% endraw %}
 
 ### Multiple Covers
@@ -169,6 +178,7 @@ cover:
 This example allows you to control two or more covers at once.
 
 {% raw %}
+
 ```yaml
 homeassistant:
   customize:
@@ -249,6 +259,7 @@ automation:
           entity_id: cover.cover_group
           position: 25
 ```
+
 {% endraw %}
 
 ### Change The Icon
@@ -256,6 +267,7 @@ automation:
 This example shows how to change the icon based on the cover state.
 
 {% raw %}
+
 ```yaml
 cover:
   - platform: template
@@ -282,6 +294,7 @@ cover:
             mdi:window-closed
           {% endif %}
 ```
+
 {% endraw %}
 
 ### Change The Entity Picture
@@ -289,6 +302,7 @@ cover:
 This example shows how to change the entity picture based on the cover state.
 
 {% raw %}
+
 ```yaml
 cover:
   - platform: template
@@ -315,4 +329,5 @@ cover:
             /local/cover-closed.png
           {% endif %}
 ```
+
 {% endraw %}