Use app.add_css_file when building docs

app.add_stylesheet was deprecated in Sphinx 1.8 and will
be removed in 4.0
https://www.sphinx-doc.org/en/master/extdev/deprecated.html

The replacement app.add_css_file
https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx.add_css_file
was renamed from add_stylesheet in 1.8

docs/source/conf.py

@@ -155,7 +155,7 @@ html_title = "OpenMC Documentation"
 html_static_path = ['_static']
 
 def setup(app):
-    app.add_stylesheet('theme_overrides.css')
+    app.add_css_file('theme_overrides.css')
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.