defer custom.js loading (#11465)

This PR switches the loading of `custom.js` to `defer` because we will need the entire page to be loading until the methods in this script can even execute correctly. 

Authors:
   - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
   - AJ Schmidt (https://github.com/ajschmidt8)

docs/cudf/source/conf.py

@@ -269,7 +269,7 @@ def process_class_docstrings(app, what, name, obj, options, lines):
 
 def setup(app):
     app.add_css_file("https://docs.rapids.ai/assets/css/custom.css")
-    app.add_js_file("https://docs.rapids.ai/assets/js/custom.js")
+    app.add_js_file("https://docs.rapids.ai/assets/js/custom.js", loading_method="defer")
     app.connect("doctree-read", resolve_aliases)
     app.connect("missing-reference", ignore_internal_references)
     app.connect("autodoc-process-docstring", process_class_docstrings)