[documentation] fix url to generators.png (#3220)

* Update index.txt

* [documentation] fix path to png, add {next} link to next page

* [documentation] fix url to generators.png

frappe/docs/user/en/guides/portal-development/generators.md

@@ -22,7 +22,7 @@ We added `published`, `route` in the DocType
 
 **Note:** The field `route` is mandatory
 
-![Generator fields]({{ docs_base_path }}/img/generators.png)
+<img class="screenshot" alt="Generator fields" src="{{docs_base_url}}/assets/img/generators.png">
 
 #### 2. Added Website Generator to Hooks
 
@@ -86,5 +86,4 @@ If you add a method `get_list_view` in the controller file (job_opening.py), you
 	def get_list_context(context):
 		context.title = _("Jobs")
 		context.introduction = _('Current Job Openings')
-
 {next}