help text and link for form labels #138 (#143)

* added help for timesheet duration form field - fixes #138
* added label option to link to manual chapter
* created docu chapter "timesheet" and added duration mode docu
* allow field and form option "documentation" to link to chapters in the documentation

src/Form/TimesheetEditForm.php

@@ -94,6 +94,7 @@ class TimesheetEditForm extends AbstractType
             'csrf_token_id' => 'timesheet_edit',
             'duration_only' => false,
             'include_user' => false,
+            'documentation' => 'timesheet',
         ]);
     }
 }