[pandas] Use current pandas lecture and add another lecture that uses polars instead

[mmcky]

We currently have a lecture introducing pandas in lecture/pandas.md.

We want to:

1. take this lecture and migrate from using pandas to use the polars library
2. keep the content of the lecture the the same, but update the context of the surrounding discussion where needed to correctly reference polars methods etc.
3. call the lecture lecture/polars.md
4. add the new lecture to the _toc.yml