Mark docs template description as required

In my testing, attempts to create a docs site with no description would
fail with the error:

```
Unable to create file 'component-info.yaml'
Error message: 'collections.OrderedDict object' has no attribute 'description'
```

Better to mark it required than let the user hit the error.

plugins/scaffolder-backend/sample-templates/docs-template/template.yaml

@@ -17,6 +17,7 @@ spec:
   schema:
     required: 
       - component_id
+      - description
     properties:
       component_id:
         title: Name