Added 'availability_template' to Template Vacuum platform (#10309)

* Added 'availability_template' to Template Vacuum platform

* Fixed Syntax and default

* Removed optional parameter from example config

* Apply suggestions from code review

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

source/_components/vacuum.template.markdown

@@ -15,6 +15,8 @@ return_to_base, clean_spot, locate and set_fan_speed commands of a vacuum.
 To enable Template Vacuums in your installation, add the following to your
 `configuration.yaml` file:
 
+{% raw %}
+
 ```yaml
 # Example configuration.yaml entry
 vacuum:
@@ -22,9 +24,11 @@ vacuum:
     vacuums:
       living_room_vacuum:
         start:
-            service: script.vacuum_start
+          service: script.vacuum_start
 ```
 
+{% endraw %}
+
 {% configuration %}
   vacuums:
     description: List of your vacuums.
@@ -47,6 +51,11 @@ vacuum:
         description: Defines a template to get the fan speed of the vacuum.
         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
       start:
         description: Defines an action to run when the vacuum is started.
         required: true
@@ -117,6 +126,7 @@ vacuum:
 This example shows how to use templates to specify the state of the vacuum.
 
 {% raw %}
+
 ```yaml
 vacuum:
   - platform: template
@@ -144,4 +154,5 @@ vacuum:
             - Medium
             - High
 ```
+
 {% endraw %}