doc: update sphinx api to use add_css_file

instead of deprecated add_stylesheet

doc/devguide/conf.py

@@ -129,6 +129,9 @@ if not on_rtd:
     except:
         html_theme = 'default'
     def setup(app):
+        if hasattr(app, 'add_css_file'):
+            app.add_css_file('css/suricata.css')
+        else:
             app.add_stylesheet('css/suricata.css')
 else:
     html_context = {

doc/userguide/conf.py

@@ -135,6 +135,9 @@ if not on_rtd:
     except:
         html_theme = 'default'
     def setup(app):
+        if hasattr(app, 'add_css_file'):
+            app.add_css_file('css/suricata.css')
+        else:
             app.add_stylesheet('css/suricata.css')
 else:
     html_context = {