Fix doc building for sphinx 4.x

Sphinx 4.x has renamed add_stylesheet to add_css_file and
add_javascript to add_js_file.
This patch updates the sphinx extensions in designate to use the
new methods.

Change-Id: I71baf9abb5566908da580d0c104831dea20c9d3c

doc/ext/custom_css.py

@@ -13,4 +13,4 @@
 # under the License.
 
 def setup(app):
-    app.add_stylesheet('designate-docs.css')
+    app.add_css_file('designate-docs.css')

doc/ext/support_matrix.py

@@ -442,8 +442,8 @@ def copy_assets(app, exception):
 
 
 def add_assets(app):
-    app.add_stylesheet('support-matrix.css')
-    app.add_javascript('support-matrix.js')
+    app.add_css_file('support-matrix.css')
+    app.add_js_file('support-matrix.js')
 
 
 def setup(app):