Remove heavy_tails and Add intersphinx_mapping to the Intro Series (#349)

* update toc, config and reference

* add notice

Author: HumphreyYang

lectures/_config.yml

@@ -32,7 +32,7 @@ latex:
       targetname: quantecon-python.tex
 
 sphinx:
-  extra_extensions: [sphinx_multitoc_numbering, sphinxext.rediraffe, sphinx_tojupyter, sphinxcontrib.youtube, sphinx.ext.todo, sphinx_exercise, sphinx_togglebutton]
+  extra_extensions: [sphinx_multitoc_numbering, sphinxext.rediraffe, sphinx_tojupyter, sphinxcontrib.youtube, sphinx.ext.todo, sphinx_exercise, sphinx_togglebutton, sphinx.ext.intersphinx]
   config:
     nb_mime_priority_overrides: [
        # HTML
@@ -82,6 +82,9 @@ sphinx:
         google_analytics_id: G-J0SMYR4SG3
       launch_buttons:
         colab_url                 : https://colab.research.google.com
+    intersphinx_mapping: {
+      'intro': ["https://intro.quantecon.org/", null]
+      }
     mathjax3_config:
       tex:
         macros: