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

This reverts commit 92b7720.

Author: mmcky

lectures/intro.md

@@ -11,17 +11,14 @@ 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.