[intro] wrap intro sentence about website into an only directive for html (#141)

* wrap intro sentence about website into an only directive for html

* add tableofcontents to website only in intro

Author: mmcky

lectures/intro.md

@@ -11,14 +11,17 @@ kernelspec:
 
 # Quantitative Economics with Python
 
+```{only} html
 This website presents a set of lectures on quantitative economic modeling, designed and written by
 [Thomas J. Sargent](http://www.tomsargent.com/) and [John Stachurski](http://johnstachurski.net/).
+```
 
 For an overview of the series, see [this page](https://quantecon.org/python-lectures/)
 
+````{only} html
 ```{tableofcontents}
 ```
-
+````
 
 ```{admonition} Previous website
 While this new site will receive all future updates, you may still view the [old site here](http://rst-python.quantecon.org) for the next month.