doc: update sphinx api to use add_css_file

instead of deprecated add_stylesheet

(cherry picked from commit 95f225e8fb)

doc/userguide/conf.py

@@ -135,7 +135,10 @@ if not on_rtd:
     except:
         html_theme = 'default'
     def setup(app):
-        app.add_stylesheet('css/suricata.css')
+        if hasattr(app, 'add_css_file'):
+            app.add_css_file('css/suricata.css')
+        else:
+            app.add_stylesheet('css/suricata.css')
 else:
     html_context = {
         'css_files': [