Add Polars lecture to complement existing Pandas lecture

lectures/_toc.yml

@@ -21,6 +21,7 @@ parts:
   - file: matplotlib
   - file: scipy
   - file: pandas
+  - file: polars
   - file: pandas_panel
   - file: sympy
 - caption: High Performance Computing

lectures/pandas.md

@@ -78,6 +78,7 @@ You can think of a `Series` as a "column" of data, such as a collection of obser
 
 A `DataFrame` is a two-dimensional object for storing related columns of data.
 
+(pandas:series)=
 ## Series
 
 ```{index} single: Pandas; Series