Defer loading of custom.js (#383)

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.

xref: rapidsai/cudf#11465

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

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

URL: #383

docs/source/conf.py

@@ -190,4 +190,4 @@
 
 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")