docs: replace deprecated function ".add_stylesheet()" for Sphinx 4 compatibility

docs/conf.py

@@ -113,7 +113,7 @@ def set_rst_settings(app):
 
 
 def setup(app):
-    app.add_stylesheet('css/borg.css')
+    app.add_css_file('css/borg.css')
     app.connect('builder-inited', set_rst_settings)
 
 # Theme options are theme-specific and customize the look and feel of a theme