Use common custom js & css code (#394)

This PR switches docs to use the custom common `js` & `css` code merged here: rapidsai/docs#286

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

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

URL: #394

docs/source/conf.py

@@ -88,4 +88,5 @@
 
 
 def setup(app):
-    app.add_css_file('custom.css')
+    app.add_css_file("https://docs.rapids.ai/assets/css/custom.css")
+    app.add_js_file("https://docs.rapids.ai/assets/js/custom.js", loading_method="defer")